Here is an old Simpson Muller located at the local metal museum foundry. Its a real workhorse. 3-phase motor, single wheel. The guy said the motor went out, and they found a used one for $500.00.
And some photos that show the plows a little more clearly. Note that there is a stop that keeps the wheel elevated slightly above the bottom of the tub.
Very interesting, PatJ. Their use in making explosives explains some of the evident "over-engineering." Interesting bits of info concerning the wheels was given on page 3: " Dimensions of each wheel are 11-3/4" diameter by 2-1/2" width at the circumference. The effective weight of each wheel is approximately 100 lbs. The muller caps are flush with the side face of the mullers and are equipped with a standard grease nipple to provide for lubrication "