The 1915 - and - later style cylinders were especially manufactured to fit the 1914 - and - earlier style narrow crankcase.

The "M" prefix of the serial number and the three numbers designate this motor as a factory racer.  The "M" suffix of the serial number designates the year of manufacture, in this case, 1916.  The suffix "H" was used for 1914.  The suffix "K" was used for 1915.  In 1917, an entirely new numbering system was initiated.