Dear Pete - it was good to talk with you and Jerry!
Enclosed is a brief layout of my mixer. I did not put down the dimensions of the iron fireman because they vary in size - any coal stoker will do.
The basic ingredient for this type of mixer is an adequate gear and drive unit which will give sufficient power and low RPMs to handle stiff clay. The iron fireman coal stoker proved to be the best suited of the many items that I looked at in junkyards - I found mine in a farmer's meadow. There are still some lying around in basements where coal furnaces were operated. Besides excellent gear reduction - it has a horizontal shaft and a neutral lever to take the unit out of gear for no-load starting.
The stoker has a shear pin which prevents gear and motor damage if the clay is too hard or compacted. The auger which is normally found with the stoker is quite long - so the shaft will have to be cut to fit your bin.