They're #8 phillips oval head wood screws, but I don't know length for Mustangs right off hand. If you go to a hardware or home improvement center that sells individual screws, they're super-cheap because they're common as dirt. Buy an assortment. If you bought the following, you'd probably still get change from a $2 bill:
2) 1 1/4"
2) 1 1/2"
2) 1 3/4"
If you have a contoured heel, the first 4 would cover a thin body, and the last 4 a thicker one (like a Strat). Put the remaining two in the junk drawer and call it a love story. If no contoured heel but thin body, buy 4 each of the first two sizes - one or the other will work. Place the neck in the pocket and hold a screw against the side to check depth. For a standard Strat/Tele heel, 4 of the longest ones are correct.