Based on experience gained in the Vietnam war with the UH-1 Huey Bell identified the need for a dedicated attack helicopter. This ultimately resulted in the AH-1 Cobra, Bell model 209, nicnamed HueyCobra by them. Basically, the AH-1 is a UH-1 with a redesigned, much narrower fuselage. Indeed, the rotor system with two blade main and tail rotor, single turboshaft engine, low tail boom, simple horizontal stabilisers attached to the tail boom and tail rotor on top of the vertical fin are all very similar to those of the UH-1. This tail rotor is on the right side on most versions, but not all. The AH-1 has a narrow fuselage in which pilot and gunner sit in tandem under a single canopy, of which the sides open in two pieces. Below the main rotor are short stub wings on each side, normally with two pylons each. Under the chin of the nose is a gun turret. 

To distinguish the different versions of the Bell 209, Bell 249, Bell 309 and AH-1 versions you'll have to look at

• the number of engines
• the shape of the gear box/engine housing
• the shape of the air intakes
• the shape of the exhaust(s)
• the span of the stub wings
• the shape of the tips of the stub wings
• the shape of the top of the vertical stabiliser
• the shape of the nose
• the shape of the cockpit windows
• the number of main rotor blades
• the number of tal rotor blades
• the location of the tail rotor
• whether the skids are retractable or not
• the presence of cable cutters
• the presence of sensors at the front of the gear box housing and/or engine
• the presence of a bent in the lower side of the tail boom
• the presence of a narrow ventral fin

Even though the US Army had ventilated it needed an attack helicopter, it initially wanted a larger helicopter. Bell did not give up and developed model 209 on its own initiative. The Bell 209 has curved cockpit side windows and a clean, somewhat pointed nose. The round chin turret has one long barrel gun. The helicopter furthermore has long rectangular, somewhat canted air intakes at the side of the fuselage, below the gear box, and a large single exhaust. There is a plate-shaped rim at the top of the gear box housing. The tail rotor is located on the left side of the vertical fin, slightly below the top. The stub wings have the same cross section along the whole span, with two pylons under each wing. The most typical feature of the Bell 209 are its retractable skids.

In order to attrack a possible order form Germany Bell converted a single engine AH-1S with the four blade main rotor of the Bell 412, its tail rotor and transmission system, and named it model 249. The Bell 249 was a one-of helicopter and never saw service.

Another private venture of Bell was the model 309, named KingCobra. It can be seen as the predecessor of the AH-1J and later. The most obvious external characteristics are the longer nose, with sensors underneath, and small ventral fin. The tail rotor is on the right side like later production AH-1Gs (see below). Moreover, the tail boom is about one metre longer than that of the Bell 209 and AH-1G. The main rotor has a larger diameter and forward swept tips. The chin-mounted turret now has a single, long multi-barrel gun. Bell built a single and twin engine version of the KingCobra.

The basic AH-1S, in principle modified AH‑1Gs and AH-1Qs, was further improved in three steps. The first one led to the AH‑1P below, the second step to the AH-1E. It was first desig­nated AH‑1S(ECAS), of Enhanced Cobra Armament System. Externally, it has the upgrades of step 1 and 2, of which the flat panel cockpit windows (also on AH‑1P) and the single, long barrel gun under the chin are externally the most obvious ones.

The AH-1E Cobra is the only version with a single, long barrel gun under the nose, a single engine with a diagonal exhaust deflector and flat panel cockpit windows, so easily recognised from other version. (photo: Eddie Maloney/WikiMedia)

The ultimate US Army Cobra was the AH-1F, that was first designated AH-1S(MC), Modernised AH‑1S or AH‑1S Modernised Cobra. It has all the features and updates of the AH-1P and AH‑1E, and a few more (step 3). From the outside the characteristic longer exhaust pipe is evident, a (high tension electric) cable cutter above and below the cockpit, a small dome at the front of the gear box housing and a sensor on top of the engine, behind the gear box. A later upgrade was a new air intake, that faces up.

Cobras of the Japanese army are designated AH-1S by them, but are externally quite similar to the AH-1F of the US Army. They miss the sensor on top of th engine and the cable cutters, and have a shorter exhaust.

The first production version of the Cobra is designated AH-1G, likely because there were already UH-1A until UH-1F, at least planned. This version is generally similar to the Bell 209 demonstrator. The AH-1G has a fixed skid gear though, and a chin-mounted turret with one or two short, multi-barrel guns. The tail rotor was initially placed on the left side of the vertical fin, slightly below the top, but moved to the right later during the production run. Other differences are less clearly visible.

(Y)AH-1R was the designation of an AH-1G with an uprated engine, and the tail rotor and trans‑mission system of the Bell 212. These changes are difficult to see on the outisde, if at all. Just two development aircraft were made, and experience was used in the production version AH-1S. 

The TH-1G was a training version of the AH-1G with dual controls, so for the front and rear seat, but this not clearly visible from the outside.

The United States Marine Corps wanted the AH-1 as well, it required a version with navy systems and two engines. So the AH-1J SeaCobra was born, with a Pratt & Whitney PT6C twin-pack. To fit it in the fuselage, the engine housing is wider and higher, has wider air intakes closer to the canopy and has double exhausts at the top rear. Also, the top of the gear box housing has rounded corners, not the sharp rim like on the AH-1G. Finally, the single gun under the chin has long barrels. For the rest the AH-1J is similar to the AH-1G, with the tail rotor on the right.

Iranian AH-1Js were later modified to carry TOW missiles, so they have the sensors in the nose and modified stub wings, but lack the other features of the AH-1T.

The double exhausts of the PT6C twin-pack of the Bell AH-1J SeaCobra are obvious on this detail photo. (photo: Philipp Beckers/WikiMedia)

Next to upgrades AH-1Gs/AH-1Qs Bell produced newly built AH-1S Cobras, at first known as AH‑1S(PROD) of production(?) and from 1988 designated AH-1P. Compared to the AH-1S the AH‑1P has new cockpit windows. These are flat and also have a different frame structure. For the rest it is externally the same as the AH-1S.

The AH-1G couldn't launch the TOW anti-tank missiles. Hence the AH-1Q was developed as an AH‑1G that can store a TOW missile system with four missiles on the outer pylons. For this purpose, the tips of the stub wings look different than on the AH-1G (and AH-1J), with a sort of end plate. Additionally, the AH-1Q received two sensors under the nose, making it less streamlined than that of the earlier Cobras.

As the new weapons were substantially heavier, a more powerful engine proved necessary, leading to the AH-1S. This version also has the transmission system and tail rotor of the Bell 212, although we don't see the difference on the outside. All were first tested on a converted AH-1G, designated YAH-1R (see above). Subsequently, surviving AH-1Gs and AH-1Qs were converted to AH-1S standard. Until 1988 they were known as AH-1S(MOD), of modified. Many AH-1Ss were equipped with fiberglass rotor blades with swept tips. Others received a diagonal exhaust deflector.

Some AH-1Ss were converted to a trainer. These are designated TH-1S and do not have a gun turret; that is covered by a fairing.

The US Marines also wanted a version that could carry the TOW missles, so the AH-1J was modified to the AH-1T, with similar stub wings and sensors in the nose as the AH-1Q. It has different engines, but still a 'twin-pac' with double exhausts. It powers a wider chord, wider diameter main rotor with swept tips, and a larger tail rotor. To accommodate the larger diameter rotor, the tail boom is longer, has a distinctively bent underside and a long narrow ventral fin. Finally, the vertical fin is a bit lower and now has the tail rotor at the top.

The AH-1T+ was a one-off demonstrator that Bell built based on a perceived demand from Iran for an upgraded Cobra. While the AH-1T+ did not result in orders from Iran, it was the basis for an upgraded AH-1T, the AH-1W SuperCobra. The seperate engine nacelles attached to the side of the fuselage, with long, narrow exhausts, are the main characteristic of the AH‑1W. But note that some have the long exhaust pipes of the AH-1Z. Furthermore, the AH‑1W has a slightly streched, wider nose with 'cheek fairings'. Other modifications are a sensor on top of the gear box, chaff/flare dispensers on top of the stub wings and cable cutter above and below the cockpit.

The last letter in the alphabet is reserved as suffix for the first AH-1 with a four blade main rotor, at least that has been built in large numbers. Or actually upgraded from AH-1T and AH‑1W. This four blade rotor of the AH-1Z sits on top of a higher gear box housing, and the blades can be folded. The tail rotor also has four blades, and is placed on the left side of the vertical fin. The engine nacelles have a similar shape as those of the AH-1W, but longer, round exhausts. The final key feature is that both wider span stub wings have three storage points instead of two, two under­neath the wings and one at the tip. Other differences compared to the AH-1W are smaller, such as slight fuselage stretch, the addition of an APU, a single ball shaped sensor in the nose and multiple other sensors and antennas.