CAT (Computer Aided Transceiver) System cartridge, to connect a FRG-965 (VHF/UHF Scanner / receiver).
Run on MSX1. Mentioned in YAESU FT-767 transceiver manual. https://www.yaesu.com/jp/manuals/yaesu_m/ft_767series.pdf
MSX Magazine issue 1986 10 page 180 says; The cartridge includes not only the interface circuit but contains the ROM and RAM as well. The ROM affords 11 extended BASIC commands for loading/saving RAM files, reading AGC voltage, switching between send/receive and selecting font. The RAM is for RAM files and battery backed up so does not lose its contents when the MSX is shut down. Memorize there the program to control the wireless. By doing so, you can instantly run without starting the program each time you start the MSX. The bundled software is almost same with the one introduced in MSX Magazine issue 1985 12. With FRG-965, identical control is possible.
The applicable software is FROG TRAINER explained together with program dump on MSX Magazine issue 1985 12 page 204 to 211 and on issue 1986 12 page 228 to 232.