Purely a mechanical error. Somebody keyed 1851 not 1851O and that's how the NGC system labels it.
The fun part

is that I bought it from a dealer at the Cowtown (Ft Worth) show. He wanted midway between Greysheet bid and ask for a VF '51, so I took him up on it. Without mentioning it was actually the $30 more expensive 51O and whilst talking to him all about another crazy dealer who had a Franklin mechanical error and wanted moon money for it because it was so 'rare'.
Back to my set. I had set my expectations at 1851 through 1862 because the next tier of the set borders on (is?) impossible... the 1863-1872 business strikes mintages were somewhere between small (3K in 1870, 3,400 in 1871) and very low (21K for 1863, 22K in 1866).
So I settled for an 1867 impaired proof (PF58) PCGS population 4, 2 lower (1 PF40), 294 higher - out of a mintage of 625... I suspect a lot of resubmits. NGC has 154. So the certified population is 454 or 72.6% of the mintage... but it was good-enough as an example of the post 1862s.
Oh, and I have an 1865 3CN labelled as a 3CS (another mechanical error) as a joke.
I mean, really. Low mintage and I wanted them in XF, cleanly graded... go ahead and pull the other one. I never expected to see any of them, let alone own them for my set, but sometimes fate intervenes... I posted previously about finding an 1869 in XF40 at the PNG/Dallas show (
https://goccf.com/t/200696#1848729). That same week Heritage auctioned off an 1871.
So the hunt continues - at very low intensity - for those two upgrades, plus the 1863+ business strikes, plus the recognized varieties... which is how come you never actually FINISH a set...
-----Burton
50+ year / Life / Emeritus
ANA member (joined 12/1/1973)
Life member: Numismatics International, CONECA
Member: TNA, FtWCC, NETCC, EveryCountry (online) coin club
Owned by three cats and a wife of 40+ years (joined 1983)
Author: 3rd Edition of the Sample Slabs book,
https://www.sampleslabs.info/