I don't believe anybody has made a copy of the Colt single barrel ones in .41RF in recent years. Colt themselves did copies in .22 back a few decades ago, but didn't do a reissue .41
Referring to Wilson's The Book of Colt Firearms, I'm getting this: If the derringer's actually from Colt, it SHOULD have a serial number on it somewhere. Their derringers were numbered, from serial #1 in 1870 in the first and second models, #1 again in 1875 with the third model and numbers starting at 100 or 1001 and having a letter suffix on all the 1959-1970s .22s.
The first and second models (basically the same as made by National before Colt bought them out; where the barrel rotates down and left to open for loading/unloading, sort of in the direction of opening most double action revolvers) had serial numbers on the butt and breach end of the barrel while the 3rd model (a different design where the barrel swings in a horizontal arc in loading/unloading) had serial numbers on the underside of the barrel, on the left side of the grip frame (I'm guessing covered by the left-side grip) and writen inside the grips. The later .22 ones had the serial number stamped on the left side of the frame.