I don't know how regularly this site is being updated these days, but it could be a good starting point for some reading:
https://www.747sp.com/
I guess the fact that it was/is an offshoot of a more succesfull product has influenced its career. I can think of a couple of other low-numbers types, but none with this many operators. As
Asturias56 said: within its niche market it did very well.
Edit: there are 58 different operators listed on that website.