You have done well to find replacements. However, I suspect is another example of calling a thing a "gun screw" when it is actually something fairly standard. So far as I know, most all quality socket cap screws are Grade 8 - I believe that is "hardened" - is stronger, more resistant to shear and pull force than a hardware store Grade 5 screw or bolt. I think 1/4" x 28 tpi is pretty much standard Unified National Fine series. I had made a replacement for the front action screw on a Remington 788 from a Grade 5 UNF 1/4x28 bolt - I re-shaped the head on my lathe and then cut a flat blade screwdriver slot - then used cold blue. That bolt was perhaps $0.03 at local hardware store - was not a "Remington" part. 10-32 thread is also listed in the UNF series - specific dimensions for major diameter, minor diameter, thread pitch angle, etc. are listed on Internet. Might be easier to find them if looking for a "threaded fastener", rather than as a "gun screw"?
As you have no doubt discovered, seems that the world is awash in various size and thread pitches of fasteners - so far, I have most taps and dies for UNF, UNC, British BA, and some metric. I do not own BASF, Whitworth and various others - so especially with small screws is a real challenge to find replacements. From an acquaintance who plays with older sewing machines - some of those makers did use "one-of-a kind" threads in their machines, and will be found no where else. I think that I read that some screws used in firearms made at the British Enfield armoury are not going to be found very easily in North America - what we today call a Lee Enfield No. 1 and similar. Makes one wonder who would come up with a 37 1/2 thread per inch thread, but that is at least one pitch from that Enfield series ...
This past week, I discovered that I managed to lose the M4-0.7 metric thread screw that holds a "hand shelf" onto my Schultz and Larsen Model 61. I ran an 8-32 tap through the female thread, then cut off a 4" long 8-32 screw to length and re-installed that hand shelf - the slotted head of that screw is only thing visible - if you look directly into the tubular hole in the outboard side of that "shelf".
