There are many different variations in the construction of the camera as it went through it's production life. As cameras were not assembled in serial number order, later components are found on earlier cameras. Early cameras are marked MIOJ on the baseplate. Later cameras are stamped in the leather on the back. Early cameras are finished in dull chrome, later cameras in the bright chrome found on S cameras.
Somewhere around camera M6092350 flash synch was added to the camera design, although cameras with earlier numbers may be synched, and cameras with later numbers not. M6092350 was synched. The factory refered to synched cameras as model MS. Collectors call them synched M's.
The "M' was dropped from the top plate around serial 6094000 in January of 1951. Approximately 3200 Nikon Ms were built since the introduction in 1949, about half of which are synched.