Used to sell these when i worked at Robin, all the lads i was there with now work at or own Kewtech. I think this item was originally made by Kyoritsu but the tooling was later sold to someone in the UK. We sold 1000's to BT in the early days but later as they got a bit old hat it all dropped away.
I have put a few call in and if i have any luck with drawings i will let you know, we could check it out for you here but we don't have any diagrams either.