I am the current custodian of MT 6101, a 1929 Mulliner bodied military. The information on these vehicles is hard to find, with just one paragraph in most Austin 7 books.
In reference to the original question I have a poster entitled 'The Longbridge Produced Austin 7 Bodies 1922-39, Published by the Austin 7 Clubs Assn. 1985.'
Which reveals the following information:
1) Mulliner Bodied Military Type – March 1929, 158 Produced 1929-30
2) Later Mulliner Bodied Military Type – Early 1932, Longer Bonnet, Louvered
3) Austin Military Type – 1932 Sole restored example (No.69) in blanford forum royal signals museum, one other known.
4) PD 2 seater – Also known as Military 2 Seater, India Office Variants.
My personal understanding is as follows.
The prototypes produced by Mulliners were used to allow the military to asses the abilities of cars compared to horses and to find what they could be used for. The Austin 7 is supposed to have been chosen for this because it was cheap, readily available and billed by Austin as capable of reaching every corner of the empire.
The further developments were due to there most common role as artillery spotters coinciding with the development of the radio. The first cars returned to a lorry to relay their messages, before the type 3, as at blandford, could be fitted with a radio, rather defeating the low profile of the car.
As far as chassis numbers I think the motor heritage museum is your best bet.
Equipment carried would also interest me, there’s definitely no space for any. The boot is divided into two and I'm told the bottom part is an ammunition locker. This seems like overkill given the only armament is the two Enfields on the boot lid.