A very nice large cent. I'm pretty certain that this is die variety N-6. The second 5 is a bit deformed and the position of the date are diagnostic of N-6 which has an R.3 rarity designation.
As far as grade I'm willing to go as high as EAC 40 which would correspond to an EF-45 to AU-50 market grade. The planchet has an honest natural color.
This is a great cent that any copper collector would be proud to own.

1855 N-6 R.3 40/40