Greetings, and welcome from Florida!
The gun you have started life as a Model 1842 Springfield musket, which I believe was the last smoothbore as well as the first
percussion smoothbore issued to the US military. The 1849 date on the lockplate is the date of manufacture. These continued in use even into the War Between the States. According to Coates & Thomas'
An Introduction to Civil War Small Arms, about 250,000 (another source stated 275,000) of these were produced between 1843 or 1844 and 1855 at Springfield and Harper's Ferry, and it was the standard arm of the US Infantry until 1855. Your gun was originally a fullstock with a 42" barrel, three barrel bands, and the front sight integral with the band nearest the muzzle. In its original smoothbore configuration, there would have been no rear sight, although some of these old warriors were optimistically rifled and fitted with rear sights for the War of '61. This was an experiment that did not really work very well.
After that war, there were thousands of surplus arms sold to the civilian market, and a number of them were "sporterized," like yours. The original 42" barrel was shortened, and the stock was cut back just ahead of the lower barrel band. Usually, a bead sight was added and sometimes a ramrod ferrule was soldered to the underside of the barrel near the muzzle. This was also done to many of the surplus .58 caliber rifle muskets, which were reamed smooth. Your .69 caliber musket was aready a smoothbore. These were very popular and useful guns in their day, and there are still a lot of them out there. As noted in a post above, people now frequently call them "foragers," but as far as I know the muzzleloading .58 and .69 caliber Springfiled and Harper's Ferry guns were not issued in this cut-down configuration. I'm pretty sure these are all civilian alterations.
The NRA has a nice, short article about the Model of 1842 online:
This Old Gun: US Model of 1842 I don't think they say anything about the civilian modified guns, though.
I don't think your gun is really worth a great deal on the market. Most of these "foragers" that you see are modified .58 caliber guns, and I think the '42's may be a little less common, but I would be surprised if you would get more than $500 for it, even if it is shootable. Maybe up to $650 if you find the right buyer.
From the photos, your gun looks like it is in pretty good condition. If you are not familiar with muzzleloaders, have a competent gunsmith check it out for "shootability." If it is deemed safe to shoot, you have some good times ahead of you. You'll have a smoothbore capable of handling shot or ball that is sturdy, nimble, and easy to maintain.
Be safe, and have fun with it!
Notchy Bob