Here's a link below from Stephen Gandy's Leica M Guide.
Leica M Used Buying Guide

Here's a link below to see when and where was the camera manufactured.
Leica M's Sorted by Model and Serial Number 1954 to 1999: M3 to M6 TTL

I have a 1959 Leica M3, 1990 Leica M6 (Psudo MP if you upgrade the VF to an M7 brightline VF), 2006 Leica M7. They're all good regardless of age. This has been a problem for Leica Camera GMBH cause their trashy looking M, long as it is mechanically sound and functioning; it will shoot as good as the latest M and my 1959 M3 will barely move at 1/15 sec hand held.