I have just one new one for you, Rob. C12426 - Cat Ref 31
This means you now have 28, 30 and 31. If they're all the same style, it might be a clue as to how your contract, Ref.29 looked.
Incidentally, my C5109 Cat Ref. 15 number plate bracket showed no signs of ever having had a contract plate and other photos of this contract seem to show the same. It was the first wartime contract, extended from 2000 machines to 6000 and it occurs to me that there may only have been 2000 plates ordered and production continued before new supplies arrived.
Bearing in mind that the sister BSA contract C5110 was increased from 2777 to 8000, a similar thing may have occurred which may account for why you haven't seen one of those yet either.