This is a Yaesu FRB-757 Relay Control Box, intended to switch such
devices as an external Linear Amplifier. Many of the older amplifiers
have a keying voltage that is too high to be safely switched with the
small reed relay contacts in most modern transceivers. This box has a
keying transistor system and relay that has connectors to 13.5VDC,
the PTT contacts in the radio (or footswitch, etc.) and the radio PTT.
There are also another set of relay contacts (both NO or NC contacts)
coming out to a pair of RCA jacks, which can then directly key your
amplifier, etc. Included is the original box and paperwork with
complete schematic and interfacing instructions.

This is a very handy unit that will work with just about any
transceiver where you need to switch something with your PTT line. I
used one of these on my ICOM IC-751A HF Transeiver to key my external
SB-220 amplifier which had a 120V DC keying circuit (way too high to
key directly with the relay contacts inside the transceiver). $25 plus
shipping.