2k miles from the midwest
Bunged up fork threads
I got a great deal on a 531 frame and matching fork. I was building it up and found that none of my headsets would thread on more than a couple of threads. The frame is a British-made Falcon, so I would assume that it's not threaded french or something weird. I think the problem is with the locknut cut out. Is it possible to chase fork threads?
Yes, any decent bike shop should have a fork threader.
How old are the frame and fork? I don't know about Falcon but Raleigh used their own proprietary bottom bracket and fork threading for a long time. They were close to English/ISO but not identical so the threads wouldn't interchange.