That's not a knob. I think it was a pivot point.
Assuming it was brass, I gave the nozzle end a light twist, and it broke. It is some sort of cheap alloy, like pot metal. Oh well.
Thanks for the inputs, guys.
I believe it was part of an old weed sprayer, the kind you pump up. The hose fit the short end and the button pushed to spray. I remember my dad having one very similar when I was a kid.