I bought an M3 which had standard PC sockets installed. When I took off the top, the flash sync area came apart as described and shown for old-style sync connectors.
I don't have photos or remember exact details, but it sure looked pretty simple. Maybe there was more than I could understand going on further inside? I don't know the history of my camera, but it had new-ish shutter curtains, and it was double stroke converted to single stroke, so someone had been deep inside the camera at least once.