The yataghan blade is 543 millimeters long and has unstopped fullers.  

Jason Kaplan believes that this is a very early war Confederate bayonet for the Mississippi Rifle (Model 1841) with imported blades from S&K and hilts made by the Confederacy.  He has test fitted several of them on Confederate altered Mississippi Rifles and some fit and some were very close. 

The big issue with this bayonet is the S&K blade.  Was it made in the US with blades imported from Germany or possibly blades salvaged from surplus stock? Could the bayonet have been made outside of the US?  It is certainly in the US style and it could be made for one of the many alterations of the US Model 1841 Rifle.  

The hilt design is similar to bayonets made by Collins or Ames but this bayonet is different enough to believe it was made by another manufacturer.  Plus, It would be very surprising if Collins or Ames would have used imported blades.  It could possibly be made by P.S. Justice.  I am sure they would have used imported blades if available but the overall finish to the hilt is superior to what I have seen on Justice bayonets.

The number 2 stamped on the disk finial looks very similar to some Confederate bayonets with single digit number markings. See the third image on page 300 of Confederate & Southern Agent Marked Shotguns, by Pritchard and Ashworth, for a similar marking.  It is believed that several small shops made bayonets for the Model 1841 rifle for the Confederacy at the start of the war.  

This Confederate selection is a good possibility and with Jason Kaplan's knowledge and experience with other examples in his collection to back it up, it was decided to list it as Confederate until proven otherwise.