@HS30-H Thx for the info. Part of my confusion is that I mis-remembered the labelling on the console, and when I re-examined my pictures, the dial was labelled "HEAD.SELECT", and not "HEAD RHEO". My confusion lay in the fact that the internals of the box appeared to contain resistive wiring, making me think of it as being some sort of rheostat.
In sort, the stalk is as you described, and works as expected when the "Head Select" switch is off. It's also possible that there is a jumper cable I'm missing that somehow wires combinations of the 6 lights together in one easy-to-us switch - and that how I've got it wired now is incomplete.
I've included a closeup of the console dial here for others to look at, with the "HEAD.SELECT", so folks know what is being discussed. Sorry for the dark picture, it wasn't very bright to begin with.