Year and CC of engine

Trying to identify year and CC of a norton engine. On the front of the crankcase is stamped 18/M 111721. If correct 18 is the model code (All 650 Twins). If M is the letter stamp it's a 1957. If correct the unique identifier is 111721 making it Sept 1964. Can anyone help me out?

I see what you mean. The normal resources I look at for such things are also similarly contradictory. Is it a single or a twin? Pictures might help. No doubt an expert will chime in soon enough.

18 is a 650, the letter code finished in 1961, there was no 650's in 1957 and 111721 is late 1964 or early 1965.