Got a reply from Tesla... These are the pages we all know: page 430 - 433 and from some other source as it seems page 140 en 151
----
Hello,
It is so big history. I can only advise some webpages. It is all.
Datasheet scan in Czech (datasheet plus process of programming):
http://www.teslakatalog.cz/MH74S571.html
Programmers for this device
https://www.elnec.com/en/device/Tesla/MH74S571/
Good luck!
TESLA
----
I will actually forget about the 21V and take this material as the truth. Started translating things a bit the other day, the English translation that I have seen is a bit wobbly, but typing this in takes ages and I'm no expert in this field. What would be handy is an OCR-ed version of the text, which I could feed to Google Translate. There might be some online OCR tool, will check in the weekend.