Yep cast stainless just like Ruger revolvers are made for frame and some MIM parts. Just looks like a really thin area wear it broke,
I did a search and can't find other cases of it happening so far. Maybe I tell them I had a .410 round in it so they can't scream "reloads" .
I know my loads are right and with Unique on the Dillon 550B you will get a .2 grain variation but not enough to blow a hammer off.