I no longer have the Club's list of issue dates of registration numbers which is rightfully with the Machine Registrar, Bennet Longman. But I can confirm that of the registration letters CHA it is the HA bit which governs where the registration was issued. After they ran out of numbers for CHA they would move on to DHA, then EHA, etc. So if your 1937 registered bike was originally a DHA prefix, again this ties up with the photos being taken late in 1936. Remember that the Ariel model year started in September or October of the previous year so that the new models were available in time for the autumn motorcycle shows. The earliest 1937 model year 500cc single went out on 2nd September 1936, at a quick check of the Despatch Book.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.