According to the Fifth Edition of the Blue Book of Modern Black Powder Arms, page 42, the Signature Series (3rd Generation) Colt 3rd Model Dragoon was "also available in a steel frame, military four screw variation with blue steel backstrap, cut for optional shoulder stock, fluted cylinder, silver plated backstrap, and round trigger guard." No information was provided as to number of such guns produced.
I note the frame on your gun is not the four screw, cut grip frame military model, but the statement about the 'available variation' is confusing as to whether the variation was one single one or whether some guns had some of those features and other guns had different combinations.
The listing suggests referencing 'Colt Blackpowder Reproductions and Replicas - A Collector's Guide' for pictures and more information; perhaps the numbers you seek can be found there.