It is a simple burnishing tool I noticed one for sale on ebay today. You could make one if you had grinding and hardening setup in the workshop. The ball is just a ball bearing.

Just using the outside of a decent bearing without the intervening ball can work pretty well. I imagine more pressure will be needed just using the bearing than with an interposed ball as shown. Effectively larger width of cut equivalent with the bearing alone. Perhaps if you tilted it to use the edge chamfer the forces would be usefully less. Bearing alone device worked well enough for me the one time I tried an experiment with the shop-made one that came with my P&W B. But I’m full size machine guy, not home shop small bench mount, so can push harder.

One day I may need it for real.

Can’t see any great need for hardening. Simply making it out of one of the more obdurate steels should be fine. Only high pressure contact between ball and housing is at the side as it is moved back and forth. Flood oiling should ensure plenty of life for our levels of use. I’m inclined to wonder if the bearing pushing the ball is strictly necessary. Plain socket with plenty of lubrication might be enough. But it is important that the ball doesn’t stall.

Clive