I am looking at the 2000 Radio Shack catalog. On page 144, item 42-2495
is described thus: "Airline headphone adapter. Use your stereo
headphones on many commercial airlines. Dual 1/8" mono plug to 1/8"
stereo jack Y-cable. Fits two 1/8" mono jacks." It cost $4.99. It does
not say whether the output on the mono end of the cable is identical at
each plug, or whether one is right channel and the other left channel.

Radio Shack seemed not to have any 3.5 mm fittings or cables. All their
fittings were either 1/4" or 1/8 ", so I suppose the 1/8" jacks and
plugs were intended to be used instead of 3.5 mm hardware.