Dave I first saw these shots many years ago with a REED BUSINESS copyright mark on them, then on the Chris Hodge site. then in VR mag. and now STILLTIME. I have often had the same thoughts as you. late 1940/ early 41 seems to be the best shot at the intro of RAFxxx reg numbers. My thought with the trailer is that it was a later shot; The ambulance would appear to be part of the same main photo shoot at Cardington, but the number plate looks twisted and mis aligned- was the picture doctored at some later date ???.
In 40 years of digging I have not come any closer to finding out the inner secrets of RAF reg numbers pre 1949, other than a Kings Reg which states regarding MT once allocated a identity number must not be changed. .
All the balloon sqn trailers in my collection either have no reg number or a Middlesex reg number.
I have a shot taken at Cardington and a sqn number is appended where the reg number should be on a Sussex balloon winch and the trailer has Serial 46 signwritten on it. it is obviously a pre issue shot.
Some snippets that might be of interest. ::
Most of the Balloon sqn vehicles that survived until the end of WW2 retained their Middelsex numbers and didn't display RAF xx reg numbers.
The handful of shots that show the Crossley Rcy and the pair of GS tenders plus ambulance Ford car/van and the Fordson tractor in front of the standard roller shutter doored tender & trailer sheds were taken at the Balloon centre at HOOK in Surrey, That lineup would equate to a balloon sqn's HQ flt establishment of MT. The the station was renamed Chessington. It later became The Rehab unit for junior ranks. I was stationed there 85/6 as SNCO rehab workshop, the workshop was accross the yard from that pair of sheds. The left hand shed contained our own MT and the London MT support fleet. In mid 86 we moved out and down the road to Headley Court to amalgamate with the other Rehab unit at Headley court. Cheessington then became a war expeditionary hospital for the USA, a thousand bed hospital was moved into the 2 sheds.