Simple, a proprietary thread - so you buy set screws from them - not someone else
I think the 1/2 screws are 1/2-14
They weren't always so "proprietary", though. Used to be LOTS more choices before they were bypassed and left to wither as majority choices took the high-volume seats of a fast-growing world. Early 1950's Pittsburgh, PA still had an old-line supplier where the array of black-oxide fasteners in wooden bins if lined end-to-end could easily stretch 200 linear yards, and about five high. "Order pickers", men in their 60's - some older - could amble directly to anything Dad asked for, too.
Not JUST "memory", though they had a bit of fun and great deal more "free advertising" letting you THINK so and go off and brag of seeing it. The treacherous old buggers had cheated. The whole shebang was systematically ORGANIZED to a fare-thee-well, y'see!
Long gone, of course. Primary supplier had been re-tooled in a New York Minute, early days of War Two, to 37 mm "Oldsmobile" cannons. Moved-on after the war off the back of all those new L&S lathes, surplus, never returned to black bolts. Neither had the plant manager. Phd Registered Professional Metallurgist and a local Pittsburgh legend at it by the time I met him as a green kid.
Meanwhile, growing adoption of "Point-Of-Sale" systems and bean-counters insisting on "inventory
turns" arrived.
Nothing may be allowed to just sit, consume shelf space, tie up working capital, and wait more than a few WEEKS now, any more than bread, fresh veg, or packaged meats. Slow-movers get tossed onto "clearance sale", not restocked. See any "Big Box" or Harbor Freight and their flyers and internet blitzing. Whatever doesn't sell well soon ceases to exist at all. Consumers have voted, and merciful we are seldom. Progress. Can't afford it any other way. Just .. Deal With That.
Not the first Pilgrim in need of these ones, though. A few folks in the "hard to find" and "antique fastener" biz still make these and many more "orphans", brand-new. Just no longer in great bins. Nor "cheap", of course! Big world. Lots of niche-markets still profitable for someone to serve.
Google may need a three or four level-down dig-dig, but
can find them or near-enough to shorten and shape-tip. Orphan or even custom taps and dies still exist as well.
Personally? I'd just aside the cripple and go scarf up a few more complete toolholders. Not as if they are in such great demand as armies are fighting to the death to control the last stash of. That "merciless consumer" thing, yah?
Mind, they CAN make far more effective doorstops than 3-jaw chucks once a body moves up to decent 4-ways. Seriously. Try one and see. Industrial steel door stopped by an Armstrong-Williams LH or RH laid flat and just so? Bugger ain't goin' NOWHERE! Those were FORGED, and of damned good steel. Forget it's there? Bend the dang door and sprain yer arm before it moves. DAMHIKT!
