Kurt Sperry said "...why in the '50s did major derailleur producers such as Simplex, Huret and Cyclo produce distinct derailleur models for 3, 4 and 5 cog freewheels? ..."
In addition to the range of travel design mentioned by Neil Foddering, some manufacturers had rear derailleurs specific to 1/8" inch chain or 3/32" chain. Among the several Simplex 303T rear derailleurs I have, those marked for 3/32" chain won't run a 1/8" chain because the pulley cage is too narrow. The derailleur's chain capacity is marked on the derailleur, so it's no surprise. Some manufacturers like Cyclo Benelux had a pulley cage wide enough to accept either chain width.
Peter
Peter Brueggeman
La Jolla California USA
4peebee(at)peterbrueggeman.com