Plunger and S/A drums look similar, but the bearing used is different. The Plunger centre boss has a somewhat smooth edge to it and a larger hole in the middle. Smaller hole and more sharper edge on the S/A type. Easy to tell side by side, even the parts book diagram shows this with fair accuracy.
These sprockets come in different chain width variants to suit other BSA model ranges, so care needed if buying a used example. Pictures show Plunger Drum and later type Backplate with the beefed up cross brace. Earlier versions are a plain pressing.
My take is that Plunger and S/A versions do not simply interchange without some element of machining.
I've always adjusted my Plunger chain at its tightest point, bike on its wheels, bribe neighbour with beer to sit and hold while I grovel down and dirty.....
Swarfy.
