The reference numbering system used on this website continues Kevin Flynn’s research first published in the mid-1990s. This system has several advantages over all others, for example:

1. Numbers by die, not die marriage. Flynn numbers are assigned to each die, or side of the coin. Newly discovered coins have the potential to feature three die varieties* – one on the front, one on the back, and one on the side. The collar is the third die, and until 2015 a collar die variety had never been found. The first broken collar variety is the 1866 COL-001, RPD-005.

1. The syntax, like DDO-001, is alphanumerically correct and can be properly sorted by a database. Computers would sort the following values in this way: S1, S10, S11, S2, S20, S3, etc. This becomes a nightmare if you attempt to catalogue varieties in a database or spreadsheet, as S2 would come after S10, S11, etc.

1. Also, a search for ‘DDO-001’ will correctly return all results containing ‘DDO-001’ as expected. However, a search for S1 will return all results containing S1, S10, S11, S12, etc.

1. Flynn’s numbering system is multi-generational.

1. Numismatic researchers have a tendency to tie their last names to their variety listings, which has created a plethora of confusing cross-listings.

1. A universal numbering system allows the catalogue to incorporate all known varieties from all known researchers

1. Congruence across series. Flynn’s numbering system is used in other series, such as the Lincoln Cents. By using the same numbering system across series, collectors can transition smoothly between series without learning an entirely new cataloging system.

1. The numbers speak for themselves. Numbers like DDO-001 and MPD-001 tell you what kind of variety it is without having to go look it up. It also makes it amazingly easy to sort and query in a database.

*Technically, there are three dies: the obverse, reverse, and collar. The collar die keeps the planchet round as it is pressed. If the collar die were to be broken, and the other two dies were varieties, then it is possible that a single coin could exhibit three die varieties. What a fantasy piece!