My Toledo handle is a number 11. It has a bore slightly larger than 2-1/2", and I have dies for it that go down to 1/8" NPT. I suspect that the number 10 handle is for the smaller dies, but don't have any proof of that.
Rigid conduit was/is used a lot in exterior/exposed locations, and the tapered threads help keep water and explosive gases away from the wiring. I know for a fact that the couplings are tapered threads, as are water-tight fittings.