1855 Large Cents come with two different date styles -- one has slanted 5's and the other has upright 5's. In all of the years preceding 1855, the 5's had some degree of slant, though perhaps not to the degree as seen on the 1855's. Both Slanted and Upright 5's are found on 1856 Large Cents, then the issue is settled with the 1857, which has Upright 5's only. The end of the Large Cent denomination after 1857 sealed their fate.
According to the PCGS Population Report (as of March 2012), the Upright 5s variety is the more common of the two, though hundreds have been certified of each. No 1855 Large Cent (of either type) has earned a grade better than MS-66, and that includes all color variations. Amazingly, a handful of Full Reg Gem examples have survived to satisfy and excite collectors.