Chief, your brother-in-law 'done good'! That is one of what is now called the "C"Series 1851 Colt's Navy revolvers that first began the now famous "2nd Generation" of Colt percussion revolvers. They began production in 1971 at serial number 4201 and continued to 1978 with serial number 25099. Figuring +or- 2612 pieces a year, this one is from the 1972 run. The whole 2nd Generation thing came about when Colt's Anthony Imperato convinced the company to consider the idea and then worked with Val Forgett, Sr. of Navy Arms to get Aldo Uberti to make the basic parts that were finished and assembled by Colt in Hartford, Conn. As Colt began adding more models to their line and production was seen to be increasing, the operation was moved to the works of Iver Johnson Arms with Imperato being the new president of Colt Blackpowder Arms Company..."the same process of finishing components produced in Italy to Colt's specifications continued throughout the entire 2nd Generation, however, the frames, centerpins(arbors), nipples and screws for all 2nd Generation models were made by Iver Johnson Arms in (Middlesex)New Jersey. This from "Colt Blackpowder Reproductions & Replicas-A Collector's & Shooter's Guide" by Dennis Adler, the definitive work to date.
As for value, they vary a lot. In the past year or more I've seen the "C" models offered from $750 to $1200 depending on who has them and what they value them at. Some show up in the box, such as yours while others are sometimes transferred to wood boxes with accessories, not always the Colt made ones. Since Colt's last "suggested retail" price was $550 and nearly 30 years ago, sky's the limit, I suppose. Depends on how much you guys value it and how much someone else might be willing to pay. Your brother-in-law's revolver looks very good with only a bit of age tarnish on the silvered backstrap/triggerguard. All you have to do is find another and you can take turns doing the Wild Bill thing!
Now, if you guys just can't stand to see a good gun sit idle, every one I've had dealings with,and there's been a few, have done their best with 27 grains of FFFg. Why 27 grains and not 25 or 28...I have no idea, they just do. Call it kizmet, call it matching velocity to ball rotation, call it phases of the moon! That load cranks out about 815 fps and fairly whistles those balls. But whatever you all decide on, have fun! :thumbsup: