I thought you might be talking about the SB-315x/U. This was a combination key and on/off switch for a remote transmitter or other circuit. It also had a standard 1/4" phone jack so we could attach our own "speed keys" when we came on watch. I was on USS Kitty Hawk in the early/mid 1960s and grew quite used to these. They were fine keys by themselves.
Agree with N4CR: a key is simply a single pole, single throw switch. No idea about the extra circuitry in the NATO key, but just find the two wires that form the keyed circuit itself, connect to them and send.
I've used the incredibly heavy gray covered NATO keys before.
You remove the cover to expose a 6-terminal strip for wiring and the key line output is terminals 3 & 4