If you lined up a handful of forged lowers today, without seeing the rollmark (brand), about the only difference you might see is in coloration. Do some research and you'll find that there are about 4 major manufacturers of receivers, and a handful of smaller operations.
LMT = Lewis Machine and Tool
= LMT, Lauer(old), DS Arms, PWA, Eagle, Knights Armament, Barrett, Bushmaster
Lewis Machine & Tool Company :: www.lewismachine.net
CMT= Continental Machine and Tool
= Stag, RRA, High Standard, Noveske (old), Century (current), Global Tactical, CLE, S&W, MGI (1st batch), Wilson Tactical, (some?) Colt, Ratworx
Continental Machine Tool Company, Inc.
Stag Arms - Worldwide Leader In AR Manufacturing
(an individual corporation that was started by CMT)
LAR = some combination of the original owner's last initials
= Grizzly, Bushmaster (L Prefix), Ameetech, (?)DPMS, CMMG, Double Star, Fulton, Spike's Tactical, Noveske (new)
larmanufacturing.com - Home
largrizzly.com - Home
MMS=Mega Machine Shop
= Mega, Gunsmoke, Dalphon, POF (forged), Alexander Arms, Stinger, Spike's Tactical(old)
Mega Machine Shop :: Tumwater Washington
Plus several more proprietary and smaller operations.
(borrowed from another site)
There are a few minor differences between the brands, but nothing major. Most will be cosmetic, with a few brands doing a bit more machining than others. There will also be brands that tend to fit better with matching uppers from the same brand than with others.
But for the most part, any of them will work well for building a complete SUR.