Student or Learner
"It had been flourishing in a 50-year period between 1920-1970."
Is it OK?
I will say the same thing I said before - use "had been" like this when you are talking about the a time before another time in the past. This webpage explains it quite well I think.
You might use that phrase like this, for example:
"It had been flourishing in a 50-year period between 1920-1970. However, when I started working there in 1975, the company was almost bankrupt."