The AN/ARC-5 was a USN command set that appeared in 1943 to replace in new aircraft the original 1940 ARA/ATA command set.  It was not used in USAAF aircraft such as a B-24, but could be found naturally in the USN's PB4Y.  The AN/ARC-5 Maintenance handbook suggests that the T-18/ARC-5 was issued one for every five of most aircraft, while the T-19 through T-23/ARC-5 were issued one for every aircraft.

The USAAF had little use for a command transmitter operating from 2.1 to 3.0 MHz so its SCR-274-N never included a transmitter for that range, just as it never adopted an associated receiver for 1.5 to 3.0 MHz.  Only the USN used on a limited basis that range of command set frequencies.

The ebay T-18 is rough and missing cover screws.  It likely was in NOS condition when the ham hacker got hands on it.  Not as many T-18s were installed for naval service as were the other transmitters. (I ignore the rare T-15 to T-17 units.)

Mike / KK5F