V101M is a Vespa engine prefix/VIN code.

V101M is engine VIN number for 125cc engine used in 125n Motovespa, 125l Motovespa vespa models made through years 1961 - 1962.
P.S. This engine VIN number is for the engine block/casing and it doesn't mean that over the years and even accross the models there isn't slight modifications of this engine parts, their sizes or even internals.