As Hawk45 mentions, it's a 1/4-28 screw, but they're nothing special. Just a socket head cap screw (SHCS) you can get at most any hardware store for 30 cents or so.
The rear screw is a #10 course thread (also a SHCS), though I personally prefer to epoxy a threaded insert (also available at hardware stores) into the stock and use a fine thread SHCS instead. If you don't, the course threads going directly into the glass stock will eventually strip out.
The insert I get is made of brass, and is both internally and externally threaded, and has a cross-slot in one end so you can drive it in with a screwdriver. I drill/tap the stock, dope up the threaded insert with some epoxy, and drive it in with a regular screwdriver. It's about as easy as eating popcorn, and will last forever.
