BIG ugly, maybe useful EV Horns eBay

[Old Guy]

My WAG is that's simply the standard they(EV) chose for small format mounting, the 1-3/8 x 18 thread is pretty much their standard like the 2 bolt Altec standard, and 3 bolt jbl standard for small format.

If we go back on the timeline, for a long time, the 6HD and 8HD were pretty much the only horns they offered aside from those re-entrant contraptions they used in the Georgian and Patrician. 8HD has been used by several other companies over the years too, lotta those suckers out there.

EV did not make large format Drivers till the MF-1012 for Kustom mid to late 70's. Later sold as EV DH-1012 and Altec 906. Later a snout was added essentially a 1.4 to 2 inch adapter cast on, to create THE DH1a.

http://trade.mar.cx/US73061636

EV, Atlas, University- the 'second tier" manufacturers all used 1 3/8 screw on for small format.. Later Heppner, Gollehon , Cerwin Vega and others used that.

That horn was designed for the above mentioned Kustom/EV/Altec driver, had to have the adapter for the small format driver.