Carl, I don't think the 1-3/8" 18 TPI dimension correlates to any pipe standard, though it's certainly OD. The threads are non-tapered, and the "snout" bottoms out in the horn. There may be a standard conduit fitting that could "work".

Do any of the threaded adapters have internal expansion, or are they a fixed ID throughout? The only one I ever used was on a Leslie, where the flare rate is already scr... uhmm... less-than-optimal.

Just wondering if it might not be better to use some of the newer threaded drivers (like B&C) that allow uninterrupted expansion to a threaded horn, than to throw a kink in the works with an adapter?