WatchCraft balance screw cutters?

[mbwatch]

I've seen a few sets of these Marshall/WatchCraft balance screw cutters (labeled as such on their original box) but I don't understand how they would be used. There doesn't appear to be a cutting surface visible for undercutting balance screws. And since they are in screwdriver like shafts, is there another tool to hold the screw, besides a typical vise sort of balance screw holder? They are like the opposite of every other type of screw cutter I've seen.

[nickelsilver]

They have spring loaded tubes, and a triangular cutter inside. The tube fits over the screw, you press and turn and cut a bit of metal out of the head of the screw for poising.

[mbwatch]

1 hour ago, nickelsilver said:

out of the head of the screw for poising.

Ah ok so they don't undercut. That makes sense. Are they meant to be use on screws in situ, still on the balance wheel?

[nickelsilver]

5 hours ago, mbwatch said:

Ah ok so they don't undercut. That makes sense. Are they meant to be use on screws in situ, still on the balance wheel?

Yes. Most purists would say that you shouldn't cut the head this way, but the number of screws I've seen (on high end pieces) cut this way tells me it's common practice.