Yes, that's always a problem on old bikes, and especially in the US, where someone's likely to have tapped to suit a UNC size. I spent ages last week trying to source a bolt to hold a tie-down in a Ford Transit van, eventually deciding it must be an M11 non-standard fine thread, only to discover it's a seat belt mount, and seat belt mounts are 7/16" UNF for some bizarre and historical reason....