Presentation On Modern Doubled Dies

You might find CONECA's division of Class IX into Type 1 and Type 2 helpful in explaining the two main types of single-squeeze doubling, if you haven't referenced that already: https://conecaonline.org/the-9-clas...oubled-dies/

For the start-date of the single-squeeze era, people common say 1996/7 as the start since Denver used single-squeeze hubs when their die shop opened in 1996 and Philadelphia fully switched over the following year, 1997. http://varietyvista.com/25%20What%2...%20Dates.htm (Though there is an argument that, since single-squeeze hubbing was first developed around 1986 and testing with production dies began around that time, that 1986-1995 single squeeze doubled dies could exist if the test dies were put into production).

For the 2022-present 'explosion' there have been some Class II+VI doubled dies found, which logically should not be possible with single-squeeze hubbing. See 2023-P 25c EK VDDR-001, 2023-P 5c WDDO-002, and others. I have not yet heard a good explanation for these, so there's potentially more going on than just experimentation with die curvature.

Hmm, I didn't notice that before. I think 'hubbed' is commonly used for that.