Hello Kye

There's plenty of information on the net as to how to date early
JAP engines. But JAP was taken over by Villiers in 1957.
I can't find any dating system for later engines.

I think your engine is a C.80, but I am not sure.
It may be that they are un-stamped. Here is a good guide
to the history of the famous JAP:-

http://www.gracesguide.co.uk/J._A._Prestwich_Industries

As you know, I have dated your machine by its features and its engine.
I have dated it to the late 1950s. That's as close as I can get.
I hope a member might be able to spread more light.

Hope this helps.
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Jack