What type of motor

It would be interesting to know what this engine number relates to.Its at present in a late 72/early 73 frame no. 2102##,engine no. 20M3S 151659,gearbox no. the same as the frame.Bike displacement is 750cc,and on 1973 registration.
Bike has plenty of parts which don't seem age related.What was the original style of this bike as it is now in the cafe racer theme.
Again thanks for any info.

Shadowcat said:

engine no. 20M3S 151659

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That's late 1971. Either Andover Norton, the UK NOC or VMCC might be able to tell you which model it was from originally, same for 2102##.