+1 on most of what mkeller said.
I would add that, although heavy steel parts that they were, they were made for durability, not light weight. Back in that day (when Schwinn had a REAL, no-time-limit lifetime warranty), they didn't want bikes to come back with broken parts. So, they outsourced to parts makers with strict specifications as for durability to be followed. If the manufacturer's parts met those specs, they got the label "Schwinn Approved" stamped on them. In the late 70's (give or take), due to Japanese competition, Schwinn could no longer maintain that great workmanship/warranty AND still sell bikes. They dropped the "S.A." label, and, for a little while used lower-end European parts before succombing to "doing business like everyone else" (i.e. outsourcing to Japan). Needless to say, "Schwinn" doesn't mean what it used to.
I still have a handful of old Huret Allvit rear deraillers that I save for old redo jobs. Heck, they have STEEL jockey pulleys with serviceable BALL BEARINGS! Heavy, yeah - but they will last a lifetime.