It sounds like a Model 5.As Mr Davis said a picture would be good.The number 1 on the bottom of the crankcase little is known about it.There is a theory that after the engine had some kind of major repair,the repairer would put that there,but that is just a theory.I would like to see it.If it is a Model 5 it's sounds like it could be the later model.The fact of the letters in front of the numbers means it does sound a bit newer.I can't say any more until I can see it as if it is a different model I could be giving you the wrong information on it.
This is the model I'm thinking it is.
[Linked Image]
[Linked Image]
Let me know if it is a different model.

Here for a good time,not a long time.