Except for about 260 Smith Enterprise M14 receivers all American commercial made M14 type receivers are cast alloy steel. Norinco and Poly Tech Industries (Chicom) M14 receivers are forged. Aside from the cast versus forged issue, dimensional geometry and heat treat is important as well.
I own Springfield Armory, Inc. M1A S/N 000049. It was made in 1971 in San Antonio, TX. I do own or have owned M1A rifles at one time or another of various serial numbers (030XXX, 052XXX, 059XXX, 075XXX, 115XXX, 123XXX, 134XXX, 141XXX). One of these is a NFA registered select fire model as well. I don't lose any sleep whatsoever when it comes to the quality of Springfield Armory, Inc. receivers. You can see minor differences in the machining of the receiver over the serial number range but it does not change the fit or function of it. FYI, Springfield Armory, Inc. in Illinois began their M1A production with Serial Number 0032XX. If the serial number is under 3200 it was made in Texas.
The other issue is how many USGI parts are on the rifle whatever the receiver serial number. Commerical made M14/M1A parts work fine for most shooters. If you are building a precision M14 type then my $0.02 is to use National Match parts. Otherwise, buy a standard model M1A and enjoy it.
A very economical but accuracy enhancing modification is to shim the gas cylinder. I did this on a pre-ban M1A. It took the groups from 4.5" to 2.5" at 100 yards using surplus ammo and iron sights.