Some older Colts use .312" front takedown pins, instead of .250" like they do now and everone else always has. Some Colts have a block in the receiver that makes it impossible to use anything but a Colt proprietary bolt carrier. All Colts, even modern ones, use larger than standard fire control group pins, although essentially any factory or aftermarket trigger component you might what is available.
Colt upper, CMT lower, problem solved (at a significant cost savings).