These are the 'A''s, which were designed for audiometric testing, I think, and also for use with the Nagra tape recorders. this one has a plug that looks like it would fit right into the jack of the old Nagra recorder I uploaded above. What makes you say that a two pronged plug would be mono? The seller told me they are over 50 years old which would make them like 1960 vintage. I'm going to try to pick up another 'A' pad and then I can stick the 'E' pads on top of that.....or even a fostex t20 pad perhaps. It should be interesting to hear them anyhow. I think the driver is the same as the 'E' driver.
I thought the A model are only 5 ohm (need to ask Judi). It's a great idea to try to mount some pads over the audiometric pads.