Mid sixties 650s with "M" in the engine number?

I'm looking for information on a couple of 650 engines I have. One is stamped 18M/111707/P, which Bacon's book dates around late 64 i.e. well after the Manxman series in 61, while the other engine is stamped 18SS/M, with the "M" slightly lower than the 18SS and then a gap to the rest of the number, which is 116180/P. This second engine (on Bacon's numbers) left the factory sometime in 65.
Both engine numbers appear to be factory correct, not something stamped or restamped later.
I did find a reference from Anna on the NOC site to a small number of 650 sold in the US as Manxman in 65, but she's unable to cast any light on the "M" in my engines, so I'm hoping someone else can help? Cheers. Bill

I thought it might help to post a picture. The number looks genuine to me - I just can't figure out why it's got an "M" rather than "SS".