Anne
Having looked into it a bit more, you would be correct that, as each new aircraft type came in to service, a new AP number was issued in the 20’s. I have noticed this with administrative type AP’s

An Air Ministry order, A.819/30, issued in 1930 laid out the three-volume scheme for technical publications relating to aeroplanes and aero-engines.

Air Ministry order, A.201/34, issued in 1934, set a standard system of numbering each of the constituent parts of the three volumes issued under this earlier order. The AP number assigned to a new aeroplane or aero-engine was promulgated by an Air Ministry Order introducing the type, the suffixes which were subsequently added to identify publications dealing with later marks of the type were also published in the Air Ministry Orders.

A.M.O. A.113/35 introduced to the Service the three volume scheme for technical air publications.