I have the official Series III parts book, and also the official Lightweight version.

Snag is that my halfshaft/drive members aren't covered in either of the books.

My halfshafts have an internal screw thread for a screw-on cap which clamps down onto an O ring on the drive member. It's the O rings numbers I need. Naturally, the size of rubber is just a bit bigger than any of the lengths of nitrile rubber that I have ready for such emergencies.

try 561981

They're now on my list, thanks James. I'm sure I've seen this type on other Lightweights so it can't be that rare can it?