I've checked this morning and my tank has a brass boss soldered to each side of the tank. I had presumed that there was a strut inside the tank and these bosses were there to fix the strut to the tank sides. Each boss has seven holes: the central one appears to receive the end of a stud (this I had assumed was the internal strut but I've now checked and there is no such strut), two holes are tapped 1/4 BSF and the other four holes are filled with solder. This tank therefore has two holes for affixing the plate which prevents the rotation problem.
I should have realised that the rubbers were affixed to plates because I have a Velocette KTS (awaiting restoration!) which has the same plates still fitted to the petrol tank. I had also acquired kneepads for the Velo and they are the same size as the HRD ones, so I have just ordered a pair of Velo ones from Grove Classic Motorcycles for about £10 each. Result!
PS Grove Classic Motorcycles is a very useful source for all those proprietary parts which are common to Vincents and Velocettes and which are not stocked by VOCSC.
You could have ordered "Vincent- HRD knee pads.
If you need, I've traced the outline of the plate and boss. Just PM me a mailing address and I'll send it to yo.
Cheers,
John