SBM "MagnaVelocity" mushroom camshaft.

If you read the DC (MP) Engine book, it's actually referred to as back-spotfacing, as I said. The spot-facing is on the underside of the lifter bore. That cannot be done with a conventional spot-facing tool - there is no room to come straight in from underneath.
So the .904 arbor is passed through the lifter bore, then the cutter (facing backwards, not in the usual forward direction) is attached to the arbor, then the tool is rotated while pulling, and the spotface is cut into the underside of the bore. :)