The following information is from my eBook on Type One Liberty Head Double Eagles at http://doubleeaglebook.com/
The 1856-O is the second rarest Type One double eagle after the 1861 Paquet, and it is the single rarest gold coin struck at the New Orleans mint. It narrowly eclipses its counterpart the 1854-O double eagle in terms of overall rarity, but it is actually slightly more available in higher grades (in this case About Uncirculated-50 and higher).
STRIKE: The strike seen on the 1856-O is fairly similar to that seen on the 1854-O. The curls around the face are well detailed but those below the ear are weak. The hair at the top of Liberty’s head is always weak as is the bun. The central reverse is well detailed with the exception of the wing tips and tail feathers which are not fully formed.
SURFACES: With the exception of a handful of pieces, every 1856-O is heavily abraded. These marks tend to be very deep and detracting. An example which is minimally bagmarked is extremely desirable.
LUSTER: As one might expect on an issue with such a small original mintage, the luster is prooflike. It is often impaired due to either excessive abrasions or harsh cleaning(s). Not many 1856-O double eagles have been spared at least one light to heavy cleaning at one time.
COLORATION: The natural coloration is a medium to deep green-gold. A few have rich yellow-gold overtones. There is probably not more than a small handful known that have original color.
EYE APPEAL: The 1856-O has below-average eye appeal. The typical piece is well worn, heavily abraded, and has been cleaned once or more. As with the 1854-O, this is such a rare and desirable issue that the concept of eye appeal has to be applied differently than on a less rare issue.
INTERESTING VARIETIES: All 1856-O double eagles have an Upright 5 in the date. There are no die varieties.
PROOFS: A remarkable prooflike SP63 1856-O exists (see Condition Census below). I formerly felt that this coin was an actual Specimen or Branch Mint Proof, but I no longer feel that this is true. A few years ago, I had the opportunity to closely view the finest known collection of Liberty Head double eagles, formed by a well-known Midwestern specialist. Included in these coins was a lovely PCGS AU58 which was fully prooflike. Immediately, I made the following conclusion: ALL 1856-O double eagles were prooflike at one time and the two really choice pieces which exist show this amount of reflectiveness because they have little or no wear. The so-called SP63 1856-O, I now conclude, is merely an outstanding first strike, and not a coin which received multiple blows of the die on a specially prepared polished planchet.
HOARDS: No known hoard of this date has ever appeared.
BUYING TIPS: You cannot be ultra-finicky when it comes to the 1856-O. Generally speaking, examples seldom come to market (maybe at a pace of one per year to year and a half). Consider any chance to purchase a decent looking 1856-O to be a special opportunity.
AUCTION RECORD: The current record price for this date was set by Heritage 5/09: 1989, graded SP63 by NGC, which brought $1,437,500. This exact coin had last sold for $542,800 as Heritage 6/04: 6372.
FINEST KNOWN: The undisputed finest known 1856-O is the famous NGC SP63 which last sold for $1,437,500 as Heritage 5/09: 1989. The second finest known is a PCGS AU58 in the Crawford collection which is ex Eliasberg: 889. The third finest is a PCGS AU55 in a New England collection, which I sold in 2009.
TOTAL KNOWN: 30-40
Very Fine: 12-15 (this includes a number of cleaned or damaged coins which will not be numerically graded by PCGS and NGC)
Extremely Fine: 9-14
About Uncirculated: 8-9
1. Private collection, ex Heritage 5/09: 1989 ($1,437,500), Heritage 6/04: 6372 ($542,800), ADD. Graded SP63 by NGC.
2. Midwestern collection. Probably ex Eliasberg: 889. Graded AU58 by PCGS. PCGS shows a second coin graded AU58 by PCGS which may or may not be a repeat submission of coin #2.
3. New England collection, obtained via private treaty in December 2007 from Doug Winter. Graded AU55 by PCGS.
4. Private Collection, obtained via private treaty from Doug Winter in 2008, Ex Heritage 7/05: 10399 ($431,250), Bowers and Merena 10/99: 1711 ($105,800; as PCGS AU53), Harry Bass collection. Graded AU55 by PCGS.
5. Heritage 10/08: 3018 ($576,150), ex Baltimore collection, Bowers and Merena 3/07: 5597 ($356,500; as PCGS AU55). Graded AU58 by NGC.
6. Heritage 7/02: 9472 ($132,500), ex “Midwestern Collection.” Graded AU55 by NGC.
One of the coins listed above is likely the ex: Amon Carter example.
POPULATION FIGURES: As of the beginning of 2015, PCGS had graded two in AU50, one in AU53, four in AU55, and one in AU58 for a total of nine in About Uncirculated. In all, they have graded a total of 15 1856-O double eagles. NGC has graded one in AU50, one in AU55, and two in AU58 for a total of four in About Uncirculated. In all, they have graded eight 1856-O double eagles. These figures are likely slightly inflated, especially in AU55 and AU58. The highest graded example with a CAC sticker is a lone AU50.
PERFORMANCE SINCE 2002: In the current market, a Choice Extremely Fine example of this date (equivalent to EF45) would sell in the $300,000-350,000 range. In 2002, this same coin would have had a market value in the $80,000-100,000 range. In the current market, a choice About Uncirculated example of this date (equivalent to AU55) would sell in the $475,000-575,000 range. In 2002, the same coin would have had a market value in the $125,000-150,000 range.
COMMENTS: I have debated back and forth for some time about the relative rarity of the 1854-O and 1856-O double eagles. My current take on the rarity level of these two issues is that the 1856-O is probably just a bit rarer than the 1854-O in terms of the total number known to exist in all grades. The 1854-O, however, is clearly a rarer coin in high grades. In this case “high grades” refers to coins in About Uncirculated-50 and above. Careful readers will note that in the “rarity” section above, I have suggested that there may be two Uncirculated 1856-O double eagles known. There is a coin currently in an AU58 holder (#2 in the Condition Census above) that I believe would regrade as an MS61 today if it were resubmitted to either PCGS or NGC.
David Akers (1975/88):
This date is one of the classic rarities of the Double Eagle series and is certainly one of the most famous and popular. It is the rarest O-mint $20 (although not really all that much rarer than the 1854-O) and, except for the 1861 Paquet and 1882 business strike, it is the rarest regular issue Liberty Head Double Eagle. In terms of number of specimens known it is more rare than the heralded 1870-CC (although it usually comes in higher grade than the 1870-CC) and is about the same rarity as the proof-only 1883. There is one very nice uncirclated piece known that was sold by Superior in 1980 and at least two others that grade Almost Uncirculated, including the Eliasberg coin which grades AU-55. The rest of the known population grades only VF or EF. All known specimens have semi-prooflike or prooflike fields.