Germane, perhaps:
http://wouter.bbcmicro.net/hardware/...rs/taxan_3.jpg
https://i.warosu.org/data/vr/img/001...9889452751.jpg
It's either:
An 8-pin EIAJ jack, used for older analog (composite video) for VTR to TV interface.
==== OR ====
An 8-pin EIAJ jack, used for RGB input - R - G - B (TTL) and sync inputs.
Yours seems to be the latter. Byte magazine used to review combo TV/monitor sets in the 80s - it may be a good source for data, as American Radio History has scans.