The IIIc was a Leica-compatible screw-mount rangefinder, 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 the IIIc was mounted with a 50mm Serenar.
Many IIIc's are still in service, and are similar in function to the Leica IIIf. There was a top speed of 1/1000, no flash sync mechanism, 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 IIIc's came with a spring-loaded takeup spool that most photographers found easier to use than the one provided by Leica -- many IIIc's are now missing the spool! since the spools were taken for use in Leica cameras.