The only .45 ACP Colt SAA I could find in my Blue Book was what was called the "U.S. Model" that was produced from 1991-1992 but it was a custom engraved model that came in royal blue finish and the shortest barrel length was 4 3/4 inches.
Colt did make models with interchangeable cylinders, so it's not hard to find a cylinder in .45 ACP. They made a Sheriff's Model with a 3" barrel and a Storekeepers mode with a 4" barrel, but neither of them had an ejector rod.
Looks like this is a gunsmith custom job as far as I can tell, but I'm sure there are some Colt guys out there that may know where it came from.