I've recently picked up a very nice older rifle--a Schultz & Larsen M68DL in 7x61 S&H. However, the action screws (which are metric M6x1) have less-than-pristine heads, and I'd like to be able to replace them. Does anyone know of someone who makes screws for guns? I've checked out a couple of large-scale manufacturers of fasteners, but haven't found anything on their websites that would duplicate the appearance of the existing screws. Here’s a picture of them. Any help will be much appreciated.

Why not just file the heads down to clean them up?

I'll teach you how to restore screws. Do Not file on them; that removes metal you need.
Put the screw in a vise, head up. Peen the burred slot edges back where they came from.
Take a thin file, or if you have a milling machine, a slotting cutter, and cut the slot clean again.Then you can file on the top, but a belt sander is better; or use abrasive paper on a file to clean up and polish the top.
Lastly, take a propane torch and heat it until it turns blue. Carefully, if you go past blue it will turn gray; you can start over; polish and do it again.
No need to make a new one.
If I make you new ones, it will cost you $20 each.

Those are definitely salvageable. Ditto dpcd.

Well...If you want to go with allen head, Brownells sells a blank guard screw kit...just wind on a proper die, turn head to proper diameter (can be doe with a drill motor) and you'll not have to worry about buggered up slots again.

brownells offers a slotted version of those screw blanks too