I sold and serviced 6300s (and 7300s, and a few non-AT&T machines)
at my first job.  I really liked those 6300s.  They were built by
Olivetti, who did a great job on them...they were built like tanks!
 
Xerox also sold these although i cant remember their model no.  But they ran GEM and
Ventura DTP very well (although slowly) on the hires screen.
roger