L1A1 long handguard screw

[waco16]

It's not too difficult to make the screw from scratch provided that you have access to a lathe. It's only a piece of bar turned down to size with thread cut on the end. Some people get excited about the idea of producing the slot for the screwdriver because they don't own specialist slotting/milling machines. But it's not difficult to cut the screwdriver slot with a hacksaw followed by a needle file.

Eclipse used to make a wonderful tool called the '4S' a handle with a multitude of different blades for slotting, scraping, slitting and sawing - the slotting/slitting blades came in a variety of widths and are perfect for producing screwdriver slots.
You can often find them 'on that site' for very little