I remember there was a thread on the the 4 companys that made all the lowers. Are there still only 4 that make them all?

No, there are dozens of companies that make lowers.

Maybe only four companies make the raw forgings but there are more than four companies turning forgings into lowers.

Here's an incomplete list of FORGING sources with their particular forge mark:

A (splintered) = Anchor Harvey Aluminum
AF = Alcoa Forge
C AF = Colt Alcoa Forge
C MB = Colt/ Mueller Brass
Cardinal (stylized bird head) = Cardinal Forge
CH = Colt Harvey Aluminum
CK = Colt/ Kaiser Aluminum
CM = Colt/ Martin Marietta
D (stylized) = (Diemaco) Colt Canada
DK = Diemaco/ Kaiser Aluminum
F keyhole = FNMI/ Cerro Forge
FA = FNMI/ Anchor Harvey
Keyhole = Cerro Forge

So just from this, we see at least 8 different forges that produce AR lower raw forgings.  And there are now a LOT of firms that are set up to mill forgings into finished lowers.  Some simply buy the CNC data from someone else and the computer and mill do the rest, while others go a bit farther, with broached magwells and other rather expensive (but I think rather important) touches.  Suffice to say that the forge mark will ONLY tell you that the raw forging was made by a quality forge; you have to know who did the machining to know anything more.  Fortunately, the "well known" vendors stand behind their products, and there's pretty stiff competition for the market so "better" is the direction just about everybody's lowers (and uppers) are going.

Is THIS the thread you are looking for?

Yes that was the thread I was looking for. Thanks much for all the help and answers. So far I have 2 lowers CMMG and they work great. Was thinking of picking up a couple more and thought I might try another company.

Mega and Aero are very high quality

Almost every lower available work fine. The reality is that the chief differences between standard-pattern lower are the markings.

read mills for or has milled for.


LMT (Lewis Machine Tool) - mills for
LMT, Lauer, DS arms, PWA, Eagle, [Armalite ?], Knights Armament, And Barrett

CMT (Continental Machine Tool) - mills for
Stag, Rock River Arms, High standard, Noveske, Century, Global tactical, CLE, S & W, MGI, Wilson Tactical, Grenadier Precision

LAR- mills for
LAR, Bushmaster, Ameetec, DPMS, CMMG, Double star, Fulton Armory, Spikes tactical (CCR), Grizzly

JVP- mills for
Double Star, LRB, Charles Daly

JD Machine (Kaiser Defense) mills for
Spikes Tactical (CAK)

Mega Machine Shop (MMS) - mills for
Mega, GSE, Dalphon, POF, Alexander Arms, Spikes Tactical (BMT)

Olympic- mills for
Olympic, SGW, Tromix, Palmetto, Dalphon, Frankford, Century (old)

Sun devil was designed by TKS engineering

Sonju Industrial mills
SI Defense

Superior arms mills for
Superior, Lauer (new), Defensive Edge/CY6, Del-Ton

SMOS mills for
SMOS, Larue, custom

Aero Precision Inc. mills for
Aero Precision, Spikes Tactical (COLE), Surplus Arms & Ammo

Territorial Gunsmiths mills for
Territorial Gunsmith’s, Six Sigma Arms, CY6

Tactical Innovations mills for
Tactical Innovations, Global Tactical

Essential Arms mills their own

Hughes Precision mills their own

A O Precision (was Advanced Ordnance) mills for
FNMI, Spikes Tactical (RM)


Sabre Defense milled their own

Colt mills their own

Yankee Hills Machine mills
YHM, Spikes Tactical (MIKE)
__________

Not complete but pretty much what I have been able to authenticate

How have you authenticated the list? That's the same list that I stole from a post on AR15Armory.com with a few additions & changes. It looks pretty accurate, though.

Where is the forge mark on a lower? Do they get machined off? I don't think I've ever seen one.

You have now... if you can find it...

The newer Cerro Forge lowers, as depicted above, have their mark on the right side of the mag well.