Who and what are BEL/Bell Labs amplifiers

I have noted several times the existence of a company called BEL or BELL Labs and their amps seem to be geared towards the high end market, and yet, I cannot find any sort of information about them, just the fact that they sometimes appear in the used market. What sort of company is or was BEL/BELL and what is the typical sound and look of their units. Are they simply a relic or are they just some very well kept secret. Is there are website with detail description of them?