I have these two sling swivels, the one on the left fits my no4mk1 just fine, but the devil on the right has the same thread and is otherwise masquerading as a no4mk1 swivel but the threaded portion is 10 thou more diameter and won't even begin to thread into the rifle.
Is it possibly off some model as an enforcer that might have used a slightly larger diameter trigger guard front screw?
The lefty measures .240 inch and the righty is .250 inch.
Have you tried it on a No1 action? The majority of these swivels were threaded either for No4 or No1.
If it will not fit a No1, see if you can read the (Parker Hale?) code on the top - this should indicate which rifle it was intended for.
The one that doesn't fit may have a clue in the markings on the screw head. If it says "Rawson" it will be for a No1. It could also have a "L" over the broad arrow over "1" being a Lithgow inspection mark. I think a couple of other "competition rifle builders" marketed them as well- I'm pretty sure all of the No1 swivels made in this part of the world came out of the Lithgow factory anyway.
Haven't seen a Parker Hale one, will be interested to find out how it is marked...
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