Many thanks for your reply and thoughts - all reassuring! My concern is that the 'D' prefix might lead the VMCC inspector to conclude this is a 1934 machine! This is (in my view) impossible since the original registration is from 1929, as is the JAP engine (I0Y/S 54073HJ) on top of which I have the 2nd hand sale receipt from 1931 (see attached copy). Well, let's see what he comes up with!
The attached picture indicates the position of the frame number, right in front of the petrol tank on the LHS, just about where you describe for the two you have. How did I miss it before!!
Thanks again!