I know it's not an answer to a need for a supply of ready to go good quality screws,,
but I turn out blanks on my lathe quickly leaving the shank an appropriate machine screw dia and the head it's countersink shape
all in one tool shape cut.
Then simply thread the shank with that machine screw size coarse thread size using a die.
Hacksaw off the screw and turn & thread another. You can make them quite quickly.
The coarse machine screw thread holds beautifully in hardwood like maple and walnut with a simple pilot hole first drilled.
The head of the screw left tall can be rough slotted with a hacksaw to turn it in first while fitting,,then marked for 'clocking' the finish slot and rough height if you wish to do that.
I cut the screw slot with a jewelers saw and a coarse cut .020 wide blade. Thin by any L/R standard I'm sure. But you can certainly widen them with a slotting file if you like or taper the slot as originals were.
I just don't care for screw slots on any firearm that look like they were made to be taken down with a nickel.
I use these lathe made screws in some of my cartridge gun restoration work and many European & English guns from the 1800's and forward used this style. Butt plate, grip cap and trigger guard screws usually.
While not PC for a L/Rifle,,neither is a stainless steel flash hole liner.
I think we use enough modern conveniences in building to say we are mearly doing that,,building a rifle.