Older Colt sporters had a nut and screw type pin instead of a mil-spec push pin to hold the upper and lower together. A few years back Colt abandoned the screw pin and used the standard mil-spec from then on.
Depending on vintage, the Colt upper can be full mil-spec and drop right on any mil spec lower. If it is an older civilian upper, it may require and off set adapter pin that is readily available from any of the parts outfits.
Websites like AR15.com or m4carbine.net have sections explaining the differences on Colt made uppers and how to make the older ones fit mil-spec lowers.
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