A couple of decades ago, Colt's Manufacturing introduced a new line of 1911-type pistols intended to carry a lower price so they could appeal to a wider market. These pistols were called the M1991A1 series, came in Government model, Commander, and Officers ACP ("Compact") sizes, and were finished in either a black oxide or Parkerized finish. They also differed from traditional 1911s by having Colt's Series 80 firing pin safety. Aside from the rather pedestrian finish, they were characterized by large and fairly ugly rollmarks.
In recent years, the M1991A1 series has been upgraded. The finish is now either stainless or a brushed blue that is much more attractive than the black oxide, and the roll marks have been reduced in size and redesigned to look more like Colt roll marks. And the series designation was changed to just 1991. The M1991A1s are sometimes referred to as ORM (Old Roll Mark) 1991s, and the current style are referred to as NRM (New Roll Mark).
They're great guns, built on forged frames and slides like ALL Colt 1911s, and affordable.