3-56 & 6-48 are a couple of sizes Brownell's at least used to sell bead sights in. They used to sell a kit, may still do, which contained three sizes of beads in both brass & aluminum in the 3-56 size & only the largest size in either brass or aluminum in the 6-48 size. The kit also included taps & installation & removal tools in the form of pin vices shaped to hold the bead. Other sizes may well be encountered but suggest you try a 6-48. Realize this is not a common hardware thread size but a special gun size, used for a lot of scope mounts.
