5/16" x 18tpi is either UNC or Whitworth (BSW). Only a slight difference in thread angle and profile between the two. Could it be a damaged thread recut to take the courser?

5/16" x 24tpi is UNF.

Richard

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