From your model number P-2851, the following info was available:
P= Single Action
1st digit, 2= Colt Custom shop, black powder frame
2nd digit, 8= 45 Colt Caliber
3rd digit, 5= 5 1/2" barrel
4th digit, 1= Nickle Plated
The gun is a third generation SAA. It was special ordered through the Colt Custom Gun Shop between 1976 and 1995 (depending on actual S/N). Even though this isn't an old model, it still has considerable value. The going price in new condition is at least $2000, probably more. The black powder frame, grips (either elk horn or buffalo horn), the nickle finish, and the box all add value. If it has a tuned trigger or a number of other special order things, the value will be even more.
You can contact the Colt Custom Shop for all the features, options, and date of manufacturer.
Might I ask what you paid for this wonderful specimen?