As plate block collectors fill in the modern issues and work back towards the older issues, they wonder why these three issues are in plate blocks of 6 instead of plate blocks of 4 like all of the modern blocks they are have.
The reason is because these three commemoratives were the last of the issues that were printed by Flat Plate printing presses.
As the Bureau of Engraving and Printing moved to using rotary printing presses, the flat plate presses were used for a few lingering issues. These three were among those issues.
Rotary printing plates have the plate numbers in each corner of the printing plate. That is what most collectors are used to: a plate block of 4 with the plate number in the corner.
However, with flat plate printing presses, the plates had the plate number in the center of the pane of stamps and not in the corner. Flat plate printing press plate blocks are typically collected as a plate block of 6 with the plate number in the center.
As collectors move into even earlier blocks before Scott #772, they’ll encounter many more of the flat plate printing press issues which are collected in plate blocks of 6. However, these three commemorative stamps, #774, 796, and 836 are very common and they are collected in plate blocks of 6 too. If you wonder why they are odd compared to all of the other modern issues, now you know.