Just putting together a 79 with expansion tank and found my radiater needed a Tridon "TRC10" instead of the "TRC11" listed which is really tiny
This is a blanking cap for expansion tank versions only
Mine has the radiator with the pipe on firewall end and was originally a 1275 ( now 998)
The TRC10 is about 59 mm I think
Thought I'd add my two bobs worth