Hoping someone can point me in the right direction here. I'm looking
for stainless antenna adapter that would allow me to mount an antenna
that's normally designed to mount on a 1" stanchion but that I want to
mount onto a standard threaded antenna mount. In essence, this is a
1"-14 female to 1" nipple adapter.

I've seen lots of adapters that do the opposite-- allow you to mount a
threaded antenna onto a stanchion-- but none that go the other way.