Hi Dan – I have a Pallas mill which is 4MT but also has the rectangular recess which matches all the tooling I have with it. Some photos of the mill in my album from a couple of years back. The torque to the tooling is via a combination of the friction of the taper and draw bar and the recess. Certainly when I am doing heavy cuts with the horizontal arbour on my machine the recess is closed up and helping drive the tool. Some of the tooling I inherited with the machine was made recently with the rectangular drive element so I would try and maintain this feature.