The 7 psi cap (with 1" reach, unlike the 6 cylinder cars) was introduced at the beginning of the TR4 run, which was actually before the TR3Bs were built.
However, I can't find any reference (other than the TRF handbook) to any TR3B being fitted with the 7 psi cap by the factory; although Bill Piggott mentions that many cars are now using the 7 psi cap.
Bottom line, I don't think anyone could say with certainty that either cap was "wrong". And since the 7 psi cap gives a bit more protection against boilover, that would be my choice. (Actually, I'm running a 10 psi cap on my TR3.)