That special 8-pin interface is for inputting a 15kHz RGB source. I've got a 9" Sony CPD-9000 RGB monitor that has the same special 8-pin RGB input interface + the special external 8-pin adapter cable with wires for completeness. It is very hard to acquire this type of speciality 8-pin RGB cable interface (though as most monitors came supplied with that necessary RGB cable when bought brand new back in the day). But if it got lost or discarded over time, it's a real bitch to find one in mint condition nowdays.
Back in the early 1980s, Sony made small RGB monitors for use with their line of microcomputers, character line generators, etc. with that special 8-pin RGB input interface that was labelled "VTR" (or Video Terminal Relay). It still is present on a lot of the bigger high-end professional Sony RGB monitors.
I use the 9" Sony RGb monitor from time to time with my supergun setup...the RGB input coming from a Neo-Geo MVS mobo is super razor sharp + the fact that the monitor is a Sony Trinitron type as well.
I do have the pinout of this special 8-pin RGB input interface if you need to know which pin accepts the following five inputs: Red, Green, Blue, Sync and Ground from a true 15kHz low resolution source. ^_~