Plate block collectors are used to different plate numbers being used for the various Scott numbers. Take, say, two different stamps from the 1940s. They were printed from different printing plates (obviously) and the plate numbers used for one issue are different than the plate numbers for the other issue. All of that is straight forward stuff.
However, if you look at the 1959 series of postage due stamps (Scott #J88-J104), you’ll notice that the same plate number exists for several different Scott numbers. How can this happen?
Here is the explanation. I’m going to keep this very simple.
The printing plates were for the pink frame only. The Bureau of Engraving and Printing (BEP) would print, say, 100,000 sheets of postage due stamps at a time from a particular printing plate. The black denominations were printed from separate printing plates (more accurately, they were more like rubber mats). The mats for the denominations had no plate number on them.
The USPS would tell the BEP that it needed, say, 10,000 sheets of 1˘ postage dues, 5000 sheets of 2˘ postage dues, and so forth. The BEP would take the stack of 100,000 sheets with the frame and grab the rubber mat with the 1˘ denomination and print those. The BEP would change mat to the 2˘ denomination and print more. And so on down the line. Therefore, the stack of 100,000 sheets having the same plate number could print two or more denominations; whatever the USPS was requesting.
That’s why collecting plate blocks on this issue is different than collecting plate blocks on other US issues. The same plate number may appear on more than one denomination or one Scott number.
You should also take note that some plate numbers on some denominations differ in price. For example, a plate number on the 1˘ issue (#J89) may be very common and normally priced like all other #J89 plate numbers. However, that same plate number may have been used for, say, very few sheets of the 50˘ postage due. On the 50˘ postage due (#J99), that particular plate number may be rare and worth a lot more than the other plate numbers that were used to print #J99