Found this video about How Green Screen Worked Before Computers, very interesting and explained a lot of things I never even questioned before, check it out:
Edit: Sorry I forgot how to post a hyperlink in this forum...
It is interesting that he mentions the Quantel Paintbox, which actually was not intended to do chroma-keying, but could do so, except only with graphics that were created in it.
Chroma-keys were done long before that in most decent vision mixers, but he did not mention the most important device of all, the Ultimatte. The Ultimatte came out in the mid 70s, predating the Quantel Paintbox by about 6 years. It was an analogue chroma-key processing system that produced far better keys than the existing vision mixer systems of the time.
When they came out the Ultimatte was the hottest thing around. It's signature party trick was to be able to key someone with whispy hair and resolve just about every hair, and you could also tune out the blue or green spill light on the subject.
I do remember though, a very odd quirk of the box was that it was designed to do either blue or green only (can't remember which) and you had to swap the RGB cables around to achieve the other colour.
Even then they were very expensive, the figure of $11K comes to mind.