No, the Census Numbers were the same...but because they had been rebuilt by 1944 when the 'B' drawn up, the reference is to the AA conversions.
Here is a photo of possibly the prototype on Bedford chassis. I have a Xerox of an IWM photo of the Ford F15 version.
Note that Z 4181050 is part of the batch Z 4176716 to 4182115 to Demand T.8957 for 15-cwt 4 x 2 G/S trucks. So, this was clearly a rebuild.
Reading the text about the history of the development in 1942 shows that Vauxhall did all the work and then produced 100 sets of mountings, although series production was spread to other contractors with Vauxhalls producing all jigs and fixtures for the other companies. The reference to Dennis was from Bart Vanderveen's text. I wonder now whether other companies were involved as well in parallel production or was it a question of multiple manufacture of parts with Dennis doing final assembly?