Opps I may be wrong, I was always led to believe that the 54RR used a thin flange barrel....but I have just stumbled on to some litrature that says;
"In 1954 the fin area was increased for the A7 Star Twin, and A10 models. In the same year the A7 SS had an alloy head with a seperate inlet manifold until 1955 when it became integral. (here's the bit about the RR)..Also in 1954 the Road Rocket gained an alloy head and the famous thick flanged cylinder, which is 1/2" thick insted of 3/8", this was fitted to all the 650 models from 1958".
So it seems that the thick flange cylinder was introduced on the 54 RR then shelved for a few years?
I will do some more research on this, and if any one else can please shed some light...
As I have stated previously..no one knows everything........... (although some claim to)