Hi & welcome to the forums,
I'm not really that up on MG's but maybe can help.
It is possible that your car has some of the older thread types on it. There was a transition period from about 1953 to 1956 when the British cars were changing from BSF (British Standard Fine) to UNF (Unified National Fine) threads. If so, your grease fittings may be the older BSF 1/4 - 26 thread. The newer UNF thread would be 1/4 - 28, which is nearly the same as the USA threads. If you have the older 1/4 - 26 thread it may take searching through some old parts to find the fittings. Or hopefully, someone more knowledgeable that I can help.
D