If the plug is to be used as the B voltage portion of a power switch, that should be right if the B+ actually needs to flow through the voice coil. If there is blocking capacitor in the circuit, one side of the horn goes to ground, and the other side goes to one terminal of the blocking cap and in that case the B+ would attach to the plate and the other lead of the blocking cap- IF the radio is wired that way. it depends on the design/manufacture of the radio.
:Regarding a mono phone jack for an external horn speaker, the tip would be B+ and the sleeve the plate of output tube?