Of course all that knowledge is lost, now....
I report what in the 1960's was common knowledge. and was quite possibly known to Vincent. The "proof" is that many Vincents with stainless guards look every bit as naff as bikes with "same size ally" guards did 40 years ago.
Vincents with Vincent guards look to me right, most Vincent with replacement stainless guards look as though the front is too big. I have Birmabrights (Birmabrites?) on my bike, and in reserve a set of black steel guards (same section) that the bike left the factory with, so am not a potential customer. I merely share my observation, not apparently unique, that current replacement stainless guards don't look right, particularly at the front, which appears to be too large a radius.