Remember the safety problem with my 1934 transmitter? Two of the phone plugs is for metering and the third is for the key. If, in the heat of the moment, I get the them mixed up and plug the key into a B+ metering jack, I'll have up to 300 volts on the key.
To protect myself I tied a length of string and a plastic cable clamp to the key phone plug (any lump of plastic would work). After the key is plugged in I slide the cable clamp under the front edge of the transmitter. The string is short enough that the plug can not be removed without getting the clamp out from under the transmitter.