Being in Australia, bullet related is possible but unlikely I would have thought.
Some sort of bead /groove roller is likely, but it's a fairly light alloy casting.
The knob seems to offer very fine adjustment of the angle of the roller. A very fine rack and pinion drives a cone into a sleeve which controls the angle.
I guess I'll have to leave it in the cool mystery box.
Have vague idea of using it for a grinding jig.
-russ