Eton, ER-4 - based on the Heil Air Motion transformer.

This is one of the best sounding ribbons we've heard. Another benefit is the low crossover point capability.
Imp - 4 ohm
SPL - 88dB
Fs - 490Hz
Flange 110 x 110mm
Cut-out 90 x 90mm
Depth 15mm
Di-pole or requires a sealed chamber on back