Bob,
55 years ago when I was in high school I worked in a gas station/garage. Back then most garages did all repairs from oil/grease jobs to tuneups, brakes, mufflers. etc.. We didn't rebuild engines just carbs, distributors, pumps, etc.. I learned a little about turning a wrench. The boss did the starter/generator work. the armature machine we had would turn the comutator, then clean the grooves and then he would put it in the V growler. I believe he would turn it on and slowly rotate it which made the growling noise. As he turned it he would hold a hack saw blade on the top which was magnetized and hold the blade tight. If not, that part of the winding was dead.
Now that is my only recollection of a growler. I am sure the guys here will give a better explanation on how to use it. I may even be wrong on this. But it brought back some memories of that garage.
John