Hi Mark- Little help they wee.
; The .577 Snider wasn't even invented by 1855, let alone incorporated in an underlever fine grade side-by-side centrefire double rifle. In 1855, the most modern double-guns were the newly invented pin-fire Lefaucheax(sp) - no outside primed centrefires yet. In 1868, there were some few inside-(bennet) primed rounds as in the .50/70 and rimfires. Again, no centrefires as in the .577 Snider before 1870something. It came after the .50-70 Sharps inside primed cases.
: Now, I do believe they misprinted the dates. Perhaps they meant 1870 to 1885.
: I don't have a date on the .577 Snider to hand at this moment - perhaps COTW has this data.
: To me- the engraving and finish overall appears to be mid to late 1880's. This is a 'feeling' only.