My info is mainly the motor cycle press reports of the Trial which give a few mentions and pictures good enough to identify two registration numbers, but probably not for reproduction here though I'll have a go if I can figure out how to do it.
There's a team pic on MotorCycling p.14 of 3rd May 1939 and a close up of the centre section of one of the M20s on Motor Cycle p.589 of 4th May '39, showing a "double thickness" Burgess(?) air filter and a bulb horn below the saddle. I may try Mortons for better copies but don't hold out much hope- they tend to be unable to find lots of stuff, which they must have, due to careless filing in the past.