"1991" was just a marketing term Colt used for a less-refined version of the Series 80 Government Models with various cost-cutting measures like rubber grips, matte finishes, big ugly rollmarks and plastic mainspring housings and triggers, meant to compete with Springfield Mil-Specs and ChiCom Norincos that were underselling Colt considerably in the early '90s. As a point of interest, Colt used serial numbers on them picking up from the GI models produced during WWII.
In the early 2000s they upgraded the 1991s, giving the carbon models a nice polished blued finish, rosewood grips, a more aestheically pleasing rollmark, and a metal trigger (but kept the plastic mainspring housing). The price went up, however, and has kept going up, until the point that they are not really a "entry level" 1911 anymore, a role the various Filipino and Turkish 1911s seems to be usurping from Colt and Springfield.
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