I can't say for pre-ban status, but I have a forged DPMS post ban lower. The serial number begins with "F" and since I only bought the lower it ends with "k". I was informed that the F=Forged and the k=kit, signifying it was not a complete gun when it left the factory.
As far as quality goes, I personally prefer a forged reciever, but I've seen it mentioned in various topics, that it really doesn't matter on the lower. Reasons cited are that the upper bears the pressures of the bullet when fired, the action of the bolt/carrier; and the lower just holds the magazine, and is a platform for the upper to sit on.
I have a Colt upper on my DPMS lower and they funtion fine together.(Only drwback is there is a color(Gray-black on Jet-black) difference if that matters. I also have two Colt's and there's virtually no difference in the lowers.
I hope this helps.