IBM The Illinois by Wayland ICA No. 23 Insuline Corp.
JACO, J.H. Ambrose & Co. KAY headphones, must be quite rare, this is the only pair I have seen
Kellogg model 64B Kennedy 1500 ohms (per side, 3000 ohms total) Kollmorgen OPT
Nice real leather headband on the Kennedy set
Keefe-Gartner Corp. Phila. PA. Keystone, by American Electric 2200 ohms DC resistance
Krauss Buckeye 3000 ohm headphones, very uncommon, in fact, downright rare, I dare say having only seen this one pair. Nice deep embossing on the back of the housings, and this pair is in working condition and the sound is quite good even on crystal radio.
Heslar 2000 ohm headphones, another very uncommon set, also never seen another one apart from this set.
Some Notes on the above headsets:
Holtzer Cabot: Holtzer Cabot made several different headphones from the 1920's such as the big can Universals which were good quality with pretty good sensitivity, and the small can 2000 ohm sets pictured that had excellent sensitivity. They also made a military set that was equivalent to WE 590's.
Holleyfone: Don't know much but I have one set with aluminum cans and this set with nickel plated brass housings, 2000 ohms.
ICA: Insuline made various test equipment and the headphones were cheap consumer grade, look like they might have been made by Cannon.
JACO: Ambrose took over from Cannon, Cannon and Miller, and if you go back even further, Cannon took over brandes headphones. JACO was still making headphones last I checked around 2001, in New York State. This pair of JACO's had the earcap crimped on so you can't get inside to fix them.
IBM: I don't know anything about these headphones, look like Trimm design but this is the only pair I have ever seen of IBM magnetic type headphones.
Kellogg: The Kellogg Telephone Company also made radio parts such as these 64-B 2000 ohm headphones. They had a unique headband and were very good quiality, with good sensitivity.
Kollmorgen: These must be very rare as this is the only pair I have ever seen. and they are not on the OTB Master Headphones List. 2200 ohms
Kennedy: The Colin B. Kennedy Company made high quality radios in the 1920's and their headsets are also well made. I have only seen a few pairs as they are quite uncommon and I have seen them go for well over $200 at auctions. The ones I have seen had real leather covered headbands and cloth cord. The construction and parts are interchangeable with early bakelite Trimms and I wonder if Trimm made these for Kennedy.
Keefe-Gartner: This is a very high quality headset with douible metal headband, celluloid insulator plate on the cans, nice bakelity earcaps, and 4 magnet bars per side. They are 3000 ohms DC resistance and sound great with crystal radio. This is the only pair I have ever seen, makes them seem rare to me.
Keystone, by American Electric: Another pretty uncommon het of headphones, again the only one I have ever seen so must be pretty rare, look similar to the Western Electric 509-W headphones from the outside, but have a single coil in each housing rather than 2 coils.
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Holtzer-Cabot Headphones from Radio News, August, 1922
Holtzer-Cabot Head-Sets, from Radio News, December, 1923
Kilbourne & Clark Headphones from Radio News, September, 1922