Q. David Bowers:
The following narrative, with minor editing, is from my "Silver Dollars & Trade Dollars of the United States: A Complete Encyclopedia" (Wolfeboro, NH: Bowers and Merena Galleries, Inc., 1993)
Die work: At the Philadelphia Mint in the latter part of 1886 or early 1887, when 1887-dated dies were being prepared, a number of 1886 dies were partially resurfaced and then punched with 1887 dates, creating overdates now known among 1887 and 1887-O dollars.
Background: The 1887/6-O overdate was discovered by professional numismatist Bob Riethe in 1972, a year after Ted F. Clark's discovery of the Philadelphia version. Little attention was paid to the 1887/6-0 in the early years, but beginning with the dissemination of information from the Van Allen-Mallis book, and, more recently, the Fivaz-Stanton guide, interest has increased sharply.
The overdate is not as sharp as the 1887/6 Philadelphia coins, and in later states is very faint and quite difficult to immediately discern. The best advice for the collector is to cherrypick for quality and seek an early die impression.
Probably many regular 1887-O coins await identification as overdates, as the variety has not been widely publicized. However, like its Philadelphia Mint counterpart, it has been listed in the Guide Book for several editions.
Circulated grades: Most known examples of the 1887/6-O are in worn grades, and these are somewhat scarce. As noted, many 1887-O dollars have not been checked for the overdate feature. In addition, this feature may not be discernible on lower grade coins. Probably 7,500 to 15,000 or more worn pieces exist.
Mint State grades: Most Mint State coins are in lower grades such as MS-60, MS-61, and MS-62, and are average or below average strikes. Probably at least 1,000 survive at this plateau. A group of 400 Mint State coins, nearly all of which were in lower levels of preservation, surfaced at the 1977 American Numismatic Association convention in Atlanta, Georgia. Probably most of these are widely dispersed today. At the MS-63 position I estimate that 300 to 600 remain, a population which drops precipitously to just 60 to 120 in MS-64 and to possibly none (but in any event surely not more than 10) in MS-65 or better condition. As more and more Mint State 1887-O dollars are examined for the overdate feature, the true rarity will become more evident.
The PCGS tour of silver dollars, which took place in 1990 and 1991 and which featured some of the finest certified coins, borrowed from various collections, had an 1887/6-O in just MS-64 grade, one of the very few coins in the exhibit in less than MS-65 preservation.
1. 1887/6-O overdate: Breen-5597, VAM-3. Usually AU or worse; Uncirculateds are mostly mediocre or worse. I have heard of no prooflikes. The 1886 date was from the Even 6 logotype.
Dies prepared: Obverse: At least 1; Reverse: At least 1.
Circulation strike mintage: Unknown part of 1887-O.
Estimated quantity melted: Unknown
Availability of prooflike coins: None reported. Characteristics of striking: Usually average or below average in sharpness
Known hoards of Mint State coins: The group of 400 was sold sold at the 1977 ANA convention and, possibly, was part of the 1962-1964 Treasury distribution of 1887-O dollars.
This variety was not known to collectors until 1972. Last 0 mint overdate dollar.