Per Flaydermans 9th edition, p.89 - Colt Model 1851 Navy Revolver, ..."The number of variations is considerable, all detailed in the reference work '51 Colt Navies, by Nathan L. Swayze. Basic to classification of variants are trigger guard types, markings, contours of certain parts, materials of grip straps, and attachable shoulder stocks. Caution advised for so-called "Dragoon Variant" with part round/octagon barrel. As with M. 1849 (see details), consensus of expert opinion believes variant never made by Colt and likely all specimens spurious."
Hmmm......