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.

It's a twin.

In that case it must be a 650, late 1964 or early 1965 as described by @possm.

The interesting bit is the "M". I think there are discussions out there about what the "M" signifies, but I do not recall seeing a conclusive answer. There is no "M" on my 1962 650 SS.

M could possibly indicate its a Mercury ? These incorporated a few Commando parts machined to fit? as 650 parts ran out.

norton bob said:

M could possibly indicate its a Mercury ? These incorporated a few Commando parts machined to fit? as 650 parts ran out.

Click to expand...

Unfortunately, I believe the Mercury was 1968-1970. So such would not fit. Similar problems for Manxman, as I think that model was only available 1961-62.

S/N 111721 was built Sept/Oct 1965, so definitely for the 1966 model year and to 1965 650SS spec which was the only 650 version built at this time (the 650 standard model had been axed two years before).
Thus, normal stamping would be "18 SS / 111721". Is rest of the engine conform to a 1965 650SS ?
Assuming S/N 111721 is correct and genuine, the deviant stamping suggest the engine has been rebuilt with a new set of crankcases, maybe in 1968/69 when the Mercury was made.

Of course, the stamping could be an amateur's attempt to re-stamp an engine in order to match the frame number available. I have seen many examples of this. Often, the stamp's font style or remnants of the old stamping reveals the forgery.
In this case, you need to look for other details at the engine to identify year of manufacture.

- Knut