The early Canon rangefinders were [Leica]]-compatible screw-mount cameras, made in the early 1950's. Many were brought to the U.S. by servicemen, who purchased them while visiting Japan during the Korean war. Typically they were mounted with a 50mm Serenar.
Many of them are still in service, and are similar in function to the Leica IIIf. There was a top speed of 1/500 or 1/1000, a flash sync mechanism only on some models, and the Leica-mount flange replaced the similar but incompatible "Canon mount" of earlier Canon rangefinders. The new camera could use any Leica-mount lens.
The original ones came with a spring-loaded takeup spool that most photographers found easier to use than the one provided by Leica -- many are now missing the spool since the spools were taken for use in Leica cameras.
Photobloggers Using the Canon II/III/IV