Spotted at a local Vintage Club show during the week, a '51 Trophy. 1 of 783 built for that year.
1951 saw the Tiger 100 alloy head and barrels fitted rather than the square "generator" top end as fitted to the GP 500s. Also note the front engine plate arrangement, the down tube stops short and the "plates" are in fact a box section fabrication, this allows the dynamo to be brought forward reducing the wheelbase to 53 inches, a reduction of 2" over the 5T Speed Twin of the day.
1951 saw the introduction of the push and twist, Ceandes type filler to oil tank, replacing the previous screw on type.
The engine number shows it as early in the model year, from Engine Number 13224NA the spoked wheel symbol to denote ramped cams was used, this motor does not have that.