I think it's the 1950 update model and the barrel seat springs may be an aftermarket improvement. 1951 had the plain fuel tank.
It is a very nicely looking bike.
Does anyone know about a weakness in KH drive side crank cases?
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.