Waste of time. It will ONLY read a dead AM carrier, with the mike gain turned down, plugged into that jack.
Try it on SSB, or while modulating your AM transmit, and the digits will dance randomly.
It's the same hookup as an inline counter that gets patched into the antenna coax, but built into the radio.
Copper used to sell a "FC-250" model that had a single RCA input jack as well as the two antenna-coax sockets. Only had five digits, though.
Oh, and a counter plugged into that jack will NOT show anything while receiving.
73