It's hard to find English source. You could try machine translation of this wikipedia page [1] - but that says it was mandatory only in 1951 to 1959 and then it was changed to a recommendation.

I know my uncle was supposed to go to one company but then that didn't work our for some reason and he had to look for a job himself, which was supposed to be rare. That would be early 1980s.

[1]: https://cs.wikipedia.org/wiki/Um%C3%ADst%C4%9Bnka