Suzkat, I don't know about breech plugs--I'd think threading a large piece of steel with a brace would be a bit of a chore. But, I do know that my Great-Uncle Oscar had similar dies with which he used to thread the ends of spindles (that were part of the ginger-bread trim on an old style house) so they would mount permanently in to a base without the use of nails or screws.
Cheers, Bookie
