The Leica I (C) was made by Leitz after the initial Leica I (A), to which it added lens interchangeability. The early cameras had a varying film to flange distance, and needed the lenses to be individually matched to the body. The later cameras had a "standardized" lens mount, allowing to attach any lens on any camera body. This 39mm screw mount was subsequently used on many other camera models.
The Leica C was replaced by the Leica Standard (or Leica E).