hydraulic flat tappet lifters ??

Bighead440 said
It's a "hardened" or at least "different material" "cap" that's pressed into the base of the lifter. AC Delco lifters still have this feature on some models, and other aftermarket suppliers use those bodies with their internals. One of those suppliers calls the caps "braxed" whatever that means?

I think 'braxed' is actually 'brazed' that made it past the editor/spellcheck. If these dissimilar metal caps were an attached base as theorized, they would be more precision assembled as a friction weld like a hardened tip of a valve, ie. spin one side against the other until they weld themselves together.
Hole Saws - Braxed-Carbide, Metal Cutting [REL-HS-14HO] - $50.00 : Crowder Supply Store