Good to know. It’s mostly about barrel quality these days, and stuff like ensuring the front sight or sight base is properly clocked. I think most of the brands source the receivers from four or five companies. Much more uniform fit and finish than in years past.
You’re right - for forged receivers. There’s only something like four sources of forged receivers.
For receivers made from bar stock, well anyone can set up a CNC and EDM machine set to make receivers from 7075 bar stock. That’s not difficult.
There’s another bunch who make receivers from castings.
And then there’s the new polymer lowers.
They all work.