As Dick has mentioned, it may 'fit', but it's not the bayonet intended for this particular weapon. Are there any other markings besides the "U.S."?
Last year I purchased (internet gun vendor) a Charleville musket w/bayonet. When I received same, although the bayonet 'fit', it had British markings on in. Turns out it was for a Brit rifle make in the 1850's.
If no one can come up with an answer, you can try Shawn at bayonetconnection.com. He does charge a fee ($20) for research, and will ask for several dimensions on the blade and socket. However, the fee is refundable if you decide to sell it to him. That's what I did, and got a good reduction on the price of a real Charleville bayonet.
"No man's life, liberty, or property is safe, while Congress is in session." Mark Twain