The Colt model number (P1551) references as follows:
P=Single Action Frame
first numeral 1=Standard frame
second numeral 5=Caliber (reference book doesn't list a 5 but shows a 4 as a 32-20)
third numeral 5=Barrel length 5 1/2"
fourth numeral 1=Nickle finish or in some models, a blue finish
The serial number would be very helpful. This could be one of the rare variations or it could be a more common model. Either way, the gun has very significant value. Depending on condition and rarety, it could be as low as $1500 to as high as $25,000, maybe more if can be tied to someone famous or is a rare variation.
Your best bet would be to contact the Colt historian for a serial number search and archive letter. See:
http://www.coltsmfg.com/cmci/historical.asp Colt will provide a certificate that authenticates the gun's barrel length, grips, finish, and other features. They often can tell you who the gun was originally shipped to. The letter will cost you but it will add at least that much to the gun's value. If your gun has exactly the same features as the Colt letter, then I would contact a professional Colt appraiser. They will need to look at the gun and inspect for proof marks and inspection stamps as well as condition. They will charge you but again, it is well worth it.
