Major dilemma hold the presses, Ok, its just a sling swivel Q ... what'veIgot?

[Patrick Chadwick]

I recommend that you measure the thread PITCH
No 4 is 1/4" x 26 tpi (BSF thread)
No 1 or P14/M1917 is 1/4" x 30 tpi
My guess is that the bolt has the finer thread.
Almost all threads have a rounded crest, so finer threads will tend to fill out the nominal diameter more than the coarser thread. Before anyone grabs Machinery's Handbook and writes in to say I've slipped a cog, just put the standards on one side and measure a few bolts - it's a result of real-life manufacturing tolerances. MAX tolerance bolts must fit in MIN tolerance nuts.
It's the same with cartridge cases and chambers. But some people don't know that.

Patrick