My two cents is that it is a parts gun and maybe Confederate assembled. Anyone seriously interested in 1855 should get a copy of "U.S. Model 1855 Series of Small Arms" by John Willyard. The 1855 had many many feature changes throughout it's short life span. Made at Springfield and Harpers Ferry. Made as Rifle or Rifle Musket. Since this appears as a musket, I will only list brief description of variations of the musket.
Springfield: Type 1-brass tip, rear sight variation 1 & no box, Type2- brass tipped, small three leaf rear sight & no box. Type 3-iron tipped, small rear sight & no box, Type 4- brass tipped, small rear sight & with box, Type 5- iron tipped, small rear sight & with box, Type 1 Version 2 Altered- brass tipped, small rear sight,& no box.
Harpers Ferry: Type 1- brass tipped,long range rear sight & no box, Type 1 Version 2- as 1 except w/o any sight, Type 2- brass tipped, small rear sight & no box, Type 3- iron tipped, small rear sight & no box, Type 4- brass tipped, small rear sight & box, Type 5- iron tipped, small rear sight & box, Type 1 Version 2 Altered- brass tipped, small rear sight & no box.
Then when you get through digesting this list, Harpers Ferry lost to the Confederacy. Multiple thousands of parts and some completed were carried South to be built and modified. This would create many more sub types.
And then there was the Rifle also. Identifying an 1855 can be a real challenge.