Mind expanding on that? In other words, when you're comparing two lowers made by different manufacturers, what might you look for in determining the quality of each?
DE does have a point.
First lets eliminate the cast (not made in years) and composite lowers, we are talking about current 'mil-spec' lowers. Forged 7075 aluminum.
These are going to originate from one of about a half dozen forging plants. Raw forging go to one of about a dozen different machining facilities. All these shops use the same 'mil-spec' sheet for internal size and hole spacing/size. There are differences in things like the mag well bevel (kind of important if you play 3gun). Most are low shelf. Some get a "Gator Grip" pattern on the front (Mega). After roll marks are done these go to heat treat, then to surface finish. Damn few places do all this in house.
There are several types of so called "mil-spec finish" with some costing more than others.
The high priced "boutique shops" do not manufacture their own uppers/lowers. They are contracted out to their specs.
There are some small shops that start with raw forging and manufacturer especially nice lowers, AXTS is one, their finished product is fantastic, but you pay for it.
The machining facilities well have "seconds", these yet to be marked lowers well be sold, but to who?
Anything coming from Aero Precision, Mega, or Schneider is GTG.