"The Colt Navy can be identified by the 7.5" octagonal barrel and the scene engraved on the cylinder. (Navies with barrels shorter than 7.5" from the factory are extremely rare.) It depicts a battle between the Texas and Mexican Navies on May 16, 1843.

The first 2500 or so Navy Colts had squareback triggerguards, and some Navies were cut for shoulder stocks. One interesting accessory produced for this model is a shoulder stock which was hollowed out to serve as a canteen."

http://home.comcast.net/~dsmjd/tux/dsmjd/tech/coltperc.htm

If it is an 1851 Navy with a factory short barrel, and not cut down later, you may have a valuable piece. I would locae a knowledgeable dealer and have it appraised. Stan