I have an A-21 and headphone/mic adapter.
The adapter has the male jacks at one end for the A-21,and female
plugs for the headset and microphone at the other end. There is also a
female 3mm plug which I believe would connect to a PTT switch.
Playing around with a 3mm jack, I have found that I can make the A-21
transmit a carrier by grounding one of the jack connections, but I can
not get audio to pass to the transmitter, so I'm thinking there is
more to the PTT circuitry than I originally thought.

Can someone please describe the circuitry of a PTT switch? - Mike