I have a databook on the BT series of display chips...  
I do know I found the info on the PMAG-BA that is in my 5000/125 system (still
running Ultrix at this point)...    In fact it very well could've been the
bt459 chip, its been a while.  Wound up that the oscillator module was dead,
and its an ECL (dual outputs - one inverted) osc that I couldn't find
anywhere...  so I wound up buying a "new" BA board for $70 from some 3rd party
service place.

Anyways...  if anyone needs more info on the "bt" chips, let me know.  

Pete Hufnagel
CaptnZilog@aol.com