This is a Leica II (Model D). Or, more accurately, it's now a Leica II. It started its life as a Leica I (Model C) in 1931, according to its serial number. At a later date, the owner sent it in for upgrade to Leica II specs, which included fitting an integrated rangefinder (the Leica I didn't have one) and a standardized lens mount, among other things.
This camera is now 76 years old, yet it's amazing how much it has in common with the modern Leica rangefinders. And, which is amazing in itself, it still works.
The lens is a collapsible Elmar 5cm f/3.5, a 1955 version of the same Elmar that originally shipped with the Leica II.