Looks good, I'm not trying to be difficult here but its not 51', its a 49 or 50 model. Whats known as the early or "longstroke" model.
Went for a ride today with Mike, he used his Goldie this time. This guy is 74 and although not in good health - heart problems - bad legs etc., he can still really ride a bike. The old bugger rushed off shouting "try and keep up"

, I did but had to work at it.
Anyway to get back on topic, I asked about the year and his was definitely registered in 51. He also said that as far as he knows that model was made up to 52. Also you are right, it is a "long-stroke".