A bit of information to add to the pile. I’ve been making up fittings for these holes that will support the bobbins used for a paddock stand on a competition bike I’m building for a customer.
It’s a 1929 Flyer frame. The left hand hole is threaded for a 3/4” x 20 tpi (60deg) bolt and the hole on the right is threaded for an M18 x 1.5 bolt. I believe this is the 18mm spark plug thread pitch.
