Press Plate: Also called "printing plates" or "plate cards."
Almost all cards are made from pressing ink-coated metal plates against blank cardstock. Most printing involves the four "CMYK" colors: Cyan, Magenta, Yellow and Black. The plates all show the same photo but have different etchings depending on the color of ink to be applied. For instance, the Yellow press plate for a 1970s Pittsburgh Pirates player will be heavily etched, while the Cyan plate will barely have any etching, due to the heavy use of yellow in Pirates uniforms of that era. In the mid-1980's Topps put up many of their old plates for public auction, which was an unprecedented move and was the first time Press Plates were made officially available to collectors.
In 1997, Pinnacle started inserting the press plates used to make the base cards from the 1997 Pinnacle and 1997 New Pinnacle sets into packs and sealed cases through the 1997 New Pinnacle product. While actual Press Plates would technically feature reverse image of what is actually seen on the front or back of the card, Pinnacle stated that the press plates inserted into packs were "used in printing" and were "authentic production press plates". Thus, these press plates indeed may have been used in the printing process but may have also been some sort of color test plate.
In the recent years after 1997, press plates have appeared in Stadium Club, Topps Gallery, and other Pinnacle products and then were used extensively by other card companies through the 2000s and 2010s decades. While practically unique, each plate came in four variations, so some collectors may not consider them "True One-of-Ones.
Value for these varies greatly, depending on player, whether the plate is for the front or back, and the clarity of the etching. Some press plates are hard to identify because they are so faint in color, which is a problem most common with the Yellow plates.