Triumph went to the swinging arm frame in 1954, putting and end to production road going rigid bikes. Not so for competition stuff in the US, Dave's 1955 T100R shows a later headstock casting with a larger top bearing and a larger bottom gusset between the frame tube and the bottom bearing cup.
Also the extra foot rest lugs for the rear set pegs, this is common to the T100R and T100C and possibly the GP500 frames. Not sure how pukkah the extra brake pedal forging just in front of the rear brake stay is though, more research needed on that.
If somebody offers you a frame with these features, don't think it's been messed around with, just buy it!