It depends on how much of a purist you are about those sorts of things. Those are a standard thread screw (the size escapes me at the moment) that you can get at any hardware store for probably around 25 cents although you won't likely be able to get the EXACT same screw in appearance, just one that will work equally well. If that is good enough for you (and it would be for me) then take one of the other screws along to match up...otherwise Lone Star can probably fix you up although it's bound to cost considerably more. Any old line music store that ever sold Ludwig probably has some of these screws sitting in a drawer or jar somewhere too.

The thread size you seek is 8-32".

This is the same standardized size used by Rogers, Gretsch, Ludwig, and Slingerland drums throughout the period before overseas outsourcing.

If you are a stickler for authenticity, contact any drum collectors (no shortage of Ludwig fans here on this forum!) or stores linked to sites such as Not So Modern Drummer and ask for original Ludwig lug screws. Might be a good idea to first remove the lug missing the screw and make sure that the threads aren't stripped. Easy to do - check with another screw borrowed from one of the complete lugs. If you identify a problem you may opt to order a complete replacement lug rather than a replacement backing screw. This will save you a lot of time and trouble and will only take five minutes to do.