The one you sold me back in 2013 was a MADE IN USA AC Model RC5 (USA AC Part No 850799).
This is a Circular Top, but with an open/visible larger spring loaded seat with integral rubber washer, with a 7/8in depth to valve seat, and rated at 4psi.
The correct XK120 and early XK140 cap is of course an MADE IN ENGLAND AC Model RB5 , again a Circular Top, but instead has a closed (shrouded pressure release spring) with no sealing washer, with a 1in depth valve seat, in conjuction with a seal/gasket mounted in the radiator neck, and also rated at 4psi.
Below are photos of an original Made in England RB5 original to a 1951 Mark VII, with XK120 RB5 identical of course (and same Jaguar Part Number)
If you now have some correct NOS RB5 for sale, I would appreciate you sending one, in exchange for the incorrect NOS RC5 you sold me.
Roger