I believe there were some 18" and 20" models that were considered to be "medium" duty and had L1 spindles. I've seen a fair number of those sizes that had 5 belts. I can't imagine anyone needing any more than what the "medium" machines are capable of. Although there was the old advertisement that showed a lathe removing 500 lbs of material in an hour. That was probably done on a heavy duty model. I know I won't be turning that kind of beef any time soon.
Anyway, I was also under the impression that the later Pacemakers still used a lot of iron and were equally as heavy as the first generation. They sure don't look as cool as the older ones, though.