I'll take a swing at this one.
The musket in question is a Remington/Maynard rifled conversion of a Model 1816 69 caliber musket.
The defining parts are the large metal shelf just forward of the Maynard primer door. This was to cover the space left by the larger flintlock lockplate. The model 1855 rifle musket uses the same Maynard tape primer - but does not have that big shelf on the lockplate.
Also the large humped rear sight like that one is a larger version of the sight used on the 1855 Rifle musket.
Finally, there is no comb in the rear stock - as the model 1816's had.
E Remington and Sons got a contract in 1854 to convert 20,000 smoothbore guns to percussion using the Maynard system. The flintlock mechanisms were removed and replaced with a new lock and hammer made by Remington - with the work of alteration being done at the Frankford arsenal. The barrel had the breech end removed for the last inch and a new forged tang/breech/bolster installed in it's place. the barrel was rifled with three broad lands and grooves.
The was a strong system that could stand the higher pressures and better sealing of the massive 69 caliber minie ball. I think that they are about the second most commonly seen 1816 conversions that you see at the shows. (After the Belgian)
Hope that this helps - Gene