Nice and crusty, just as I like them, and all metal matching in age and appearance - don't clean this up, I would say.
It is a Model 1863 Springfield, dated 1863 on the lockplate. There would have been a date on top of the barrel, too.
I would say it has been stored with the bayonet in place, and so aged aka rusted that way, and I would not even remove the bayonet or you might scrape some brown off the metal and make it look uneven and no longer matching.