Wow, I cut my teeth in the biz dealing with those setups in Bergen County (Northern), NJ. I can't tell you how many of those jobs I encountered. I got to know the Square-D "QO-612" panels pretty well in fuse panel replacements. It is hard to believe that such installations still exist after my 30+ years and are sustaining today's electrical requirements. I'm sure that there are literally thousands of them out there between Baltimore and Boston at a minimum.
I remember encountering single-pole, 50 amp feeders to apartments in NYC. When was the last time that you encountered a 50 amp single-pole breaker? Oh boy, how times have changed.
