Brian:
If you'll think about it, the locking knob appears on Compacts at the time they were getting into power nozzle availability. Friction-fit wands work fine with regular tools, but you need a locking system with a power nozzle, since a power nozzle is heavy enough to slip off the wand if not locked into place. Even Lux, which stayed with friction-fit wands longer than anyone else, I think, used a locking system on its power nozzle wand. But that meant Lux was supplying two wand systems, one with the PN and one for other uses. Most vac companies would rather give you just one set of wands for use with both PN and regular tools, which means they'd have to lock somehow.