Isophon DKT-110?

I looked it up, Dave.

It's the old Isophon KK8 or KK10 1" plastic tweeter with a bullet horn added. A good tweeter in its day.

681186d1526561181-70th-anniversary-loudspeakers-briggs-mordaunt-short-pageant-jpg


Takes it up to about 100dB loudness with a honk around 5kHz.

You use a regular sort of 3.5kHz filter AFAIK. But no idea if some equalisation is needed.