Not that it matters now you have the correct one but if the one on it has mounting lugs top and bottom similar to the plunger one then it comes of a C series, if it has threaded pieces on the back that a plate bolts to then it is off a B or M, of course it may well have had pieces welded to it to mount it to the plunger.
So there is todays piece of useless information
Nice bike by the way, plunger models are my "thing", I've got a few of them.
