It is definitely a Whitney US 1812 musket, 69 cal. Those are the only ones marked that way. 42 inches is the correct length for the barrel which is a very good thing, if it had been cut down for a squirrel gun, it would have lost a lot of value. If it were in it's original flint lock configuration, depending on condition, it could be a 5-6000$ gun. I'm not sure what it would bring as a percussion conversion, but with having the nipple welded into the original barrel (I'm guessing) would make it a fairly early conversion, since later it would have been re barreled and I'd expect to see a drum on the side of the barrel. It's a really great gun you have there and I know if it were mine I'd be proud to own it.
Id love to see some better pics of the lock and barrel where the conversion was done.
EDIT : Just wanted to add : Don't try to clean it at all..... wipe it down with a soft cloth and some gun oil, but don't be too rough, you don't want to lose any patina on that great old musket...