I recently acquired this little gem. This error was caused by the 400 subject rubber plate (used to apply the denomination - much like the bureau precancels) slipping down and leaving the larger gap meant for larger space between the bottom row of the upper right pane and top row of the lower right pane to be fully in the lower right pane.
This error would also cause the same error in the lower left pane. This slippage would also then cause the top row in the upper left and upper right panes to be missing the 8 cent denomination completely (please see pane of J89 (1 cent) below).
How this can happen I can only speculate. The frame of the postage due stamps were printed first and then the denomination was added later. What I don't know were the stamps separated into sheets of 400 and then the denomination applied or did they remain as they were printed (in long rolls) and the denomination added. Long long ago (in the 1970's - before dull gum ) I once went to small post office (hunting for postage dues of course) near where I lived and the postmaster told me that she ordered sheets of 6 cent postage dues and then sent her one sheet of 9 cent ones. My heart skipped beat (actually several) and I asked if he still had it, but she said she'd sold to a local collector. Unfortunately she wouldn't give me the customer's name.
J96 27851 Lower Right sheet with wide spacing between first and second rows
J89 26660 Upper Left sheet missing denomination on the top row
The denomination was printed by a cylindrical rubber electrotype plate at a second station on the rotary press, initially designed for adding precancels. The red cross on the 1952 3 cent Red Cross commemorative was also printed in the same way. Spacing variation could occur if the second stage lost synchronization with the main printing stage and exposed the wide spacing for the gutter between panes.
Thanks for the information. I remember hearing that on some of the rubber plates had different denominations so that each pass would produce let's say the 1 cent stamps on the upper left and right and 7 cent on the lower left and right. For example this might be cause of on the 10 cent the lower left and right with plate number 35282 having never been reported.