It's 1/2 inch BSW Dave - measured 12 tpi which means it can't be UNC.

This from my 2018 car BTW, but doubt it's changed much in the last few decades.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone