Hi you lot.
I worked at SSL from 1980-87 and made some of the 681 keyboards plus lots of other stuff inside the console and most of the leads back then. There are two lots of switches on the keyboard for typing etc. I don't know the actual names of the manufacturers at the time as I just assembled various things and some manufacturers got taken over, changed their names etc (a bit like the main Highland switches back then).
I don't know the name for the smaller alpha numeric switches but, if it helps the bigger ones on the top of the keyboard were commonly referred to as 'Cherry caps'. Whether this is just those big black keycaps or not, I don't know but I think I remember the actual switches being white and green in colour although my memory may fail me over 45 years (I am now 68!).
Both sets of switches were vertical PC mount versions and there must be some modern day versions available somewhere out there surely?
If you need to have a new modern day keyboard attached instead, it may take some time and effort to link it up to the ribbon cable on the keyboard. Good luck with that and I don't know the answer.
Hope this helps from Jim Lassen, the Forum Wiring moderator based in Maidenhead, Bucks, UK and if you need DL connectors and/or SSL looms made, get in contact and check out my Facebook page link below which needs to be updated a lot!