The first model of Canon is easily identified by the large round film counter on the wind side of the front of the camera. About 75% are marked "Hansa" above the word Canon, and are refered to as Hansa Canons. These were marketed by Omiya Photo Supply Co. Others were sold direct from the factory, and are unmarked. These are refered to by collectors as Original Canons. These cameras were made concerently, and the factory considers them the same model (and they are), and designated them as "the Canon". While these cameras were inspired by the Leica, they are not Leica copies. They use a bayonet system of mounting the lenses in which the focusing mount remains with the camera, rather than the lens. Quite a lot of trouble was taken to avoid infringing on the patents held by Leitz. While these cameras are not that rare (they made about 1100 of them), they are extremely desired by collectors.
An interesting note, the lenses for most Seiki Canons, and the rangefinders and focus mounts for the Hansa and original Canons were manufactered by Nippon Kogaku, a large optical firm that later became Nikon.

The next model of Canon was an improved version of the original. They moved the counter to under the wind knob, like the Leica counter. They also added a slow speed dial (actually, more of a crank), onto the front of the camera. This camera was designeated as the "Canon Newest Model", and the original model became "Canon Standard without slow speeds" Not exactly the catchiest of names, this model became the "Standard" when the original model was discontinued in 1940. It is refered to by collectors as the Canon S. A small batch of 97 cameras were assembled after the war, and were designated as Canon S-1 cameras. However, these cameras were assembled from wartime parts, and cannot really be identified as such.

If your camera sounds like a Canon S from the previous description (bayonet lens mount, counter under the wind knob), but lacks the slow speed dial/crank, then you have the Canon NS. The factory, continuing a precident in creative naming, designated this as the "Canon new standard model without slow speeds". NS is a designation created by collectors. This is a very rare camera, total production was about 100 units.

In 1939, Canon decided to market a simpler camera to the public. In an improvment in their naming scheme, it was advertised as the "Popular Model", but was refered to at the factory as the "Junior" model, or the "J". in spite of the advertised name, they never were very popular, and prodution for the entire series is very low.
The J cameras use a lens mount, similar in diameter to the Leica, but with a different pitch. Leica lenses canbe threaded a short way onto J cameras, then they bind up. (And the same is true of J lenses on Leicas). Also, J series cameras do not have rangefinders. They can be broken down into three models:

The Canon J has no slow speed dial, and the finder cover is not curved around the rewind knob like a Leica is. Serial numbers range from about 1700 to 2125. Production was about 200 units.

The Canon JS is the same as the J, but with a slow speed dial. Serial numbers from about 1900 thru 2130. Production estmated at 50 or less units.

The J-II was the postwar version. These were built out of leftover S body shells, so they have holes drilled in them for the slow speed mechanism. This is usually covered with a metal patch, although some have been found with the holes filled, and covered over with the genuine cardboard fake leather used on these early cameras. The easy way to distinguish the J-II is that the finder cover is curved around the rewind knob, similar to that of the Leica. Production estimates range from 164 to as much as 525.

The remaining model of Seiki Canon is the S-II. This is the only camera marked "Seiki Kogaku" with a rangefinder that did not have a bayonet lens. It does have a slow speed dial. This camera was in production when the company changed it's name to Canon (it was the only model in production at that time), and roughly 2500 were built with the Seiki name.