I see the price list Dr. Drew posted is dated Feb. 10, 1881, and that rebounding locks had become "standard." The price list in my 1877 catalog has rebounding locks $3 extra on the grades up through 6.
1877 Price List.jpeg
My take is that your gun is a No. 6 with the extras of a pistol grip and rebounding locks.
Here is the page from the catalog that states the guns are chambered for brass or paper cases and mentions the chamber lengths.
1877 page 7.jpg