Early electric vintage Allen-Bradley Bradleyohm variable resistor

Old early electric and radio component... a BradleyOhm Perfect Resistor made by Allen Bradley. This is from the estate and amateur radio ham and we don't know much about it. We've been told that it is a variable resistor but we don't know how to test it.

This measures about 2" across and 2" high. There is heavy wear and some chipping to the hard rubber knob.