This Aero S2 parts book screenshot may help.

[Linked Image]

Note the locknut for TRE is shown as M14. The diameter and 1.5mm pitch is close but not a perfect match to 9/16" UNF thinking


Richard

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