Hi,
Regarding the colour of the 350 class plates, during the 50's the plates were light blue with white numbers, 500's yellow with black numbers, 250 green with white numbers. The different "grey" front number plates seen in photo's is because for the races the fronts were changed for all bikes to black on white, for the practice sessions numbers were all 3 the same colour to differentiate the machines passing the time keepers. In later years the front number was issued by the ACU and stamped so that the scrutineers could be certain that the bike had qualified. Pre war I think that the system was different, to be certain someone is going to have to dig out some pre war programmes. It may be that the TT used a different system to the Manx.
Giz, If you care to email me directly I do have a few photo's of my 90 which I built many years ago modelling it on one of the versions! that Don Chapman actually rode in the Manx, small fly screen, large tank and rev counter mounted on an aluminium disc on the handlebars. It is the bike shown in my avatar rounding quarter bridge. The black engine has never had a satisfactory explanation the truth lost in the mists of time!
Dave