This delightful photo (from the "Service and Instruction Book for 'Regent' & 'Regal' Chassis" published by AEC) shows one of the very first Regals.  According to Alan Townsin's book on the Regal, the first 12 were delivered to Plymouth in 1929 and bodies were by W. Mumford Ltd, a Plymouth body-builder.  There are photos of vehicles from this first batch of twelve on pages 5 and 6 of the book, and the vehicles are clearly the same as that shown here.

The engine was a 6-cylinder petrol version, newly designed by AEC's chief engineer John Rackham.

Incidentally, Mumfords, (founded in 1900), is still going!