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?
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
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.