Originally Posted by
turbodog
aluminum hexes
U3525 wide hex (comes with Mi4)
U2991 medium hex (comes with Mi3)
U3221 super narrow hex (option part)
U3570 ???????? don't see this one listed
here's a quick (but incomplete) synopsis: (and in chronological order)
2991 - 3.9mm wide (mi2ec through mi3.5 kits - oring style)
3221 - 3.9mm wide (slip-on, no o-ring, released for mi3)
3525 - 4.9mm wide (mi4 kit - oring style)
3570 - 3.9mm wide?? (no idea what's different)