FWIW, the cap on TS39781LO when it came to me in 1984 looked exactly like yours, Don (except for the decal and peeling paint). Obviously that doesn't prove it's original; but what are the chances of two cars almost 3000 miles apart getting the same aftermarket cap ?
Also FWIW, I don't believe that is the same "squared style" cap that the TRA guidelines say was fitted through TS17372E. It doesn't match the photo shown in the owner's manual, the diagram in the SPC, or the cap on my TS11-something TR3.