My Ameco K-4 key has a thing like a switch on it. It’s a thin metal piece with a small plastic knob and it can be rotated in underneath a flange on the lower contact. What exactly is it for?
It’s a “circuit closing switch” and not many keys have them these days. When it is closed, it is the same as if you were holding the key down.
In the early days of landline telegraphy, you had to “close your key” in order to receive signals from the other end of the line. The line was a was a continuous circuit that went through both keys and sounders.
In radiotelegraphy, the switch is not required for operation of the receiver and so most keys designed for use with radios do not have the switch. If the key does have a switch it can be used to put the transmitter into continuous “key down” for tuning the transmitter, antennna system, etc.
On ships the switch was sometimes called an “abandon ship switch.” If a ship was sinking, the radioman could throw the switch on his way out to the lifeboat, and the transmitter would send a continuous tone that would act like a beacon. It had a similar purpose on aircraft– if you have to bail out, the plane might go some considerable distance before crashing. Radio operators referred to is the “bail out switch.”
A secondary use that I have seen is to connect a bug (semi-automatic key) temporarily by inserting the “wedge” (a flat bladed plug on the end of the bug’s connecting cord) underneath the switch flange on the key. The wedge was more commonly inserted between the key contacts, but if the switch flange could be used it would not be necessary to readjust the key contacts when the wedge was removed.
In amateur radio, the customary use is to transmit a continuous signal while both hands are used to adjust the radio and the antenna tuner.