Hi Steve, as far as thread types i suspect either it was a buttress thread or an acme thread of some sort.
As far as the Altec horns go, I suspect a simple 60 degree form of thread.

Check on the more common audio forums (AK, DIYaudio), maybe there is a machinist/audio guy in your area that could help out. Post your story and some pics, someone may surprise you.

BillWojo