Hey Leon,
On the backs of uncut finished sheets the back adds would look like a large grid. Several rows and columns. It's likely this large grid was printed all at once, one strike.
If the sheet wasn't aligned properly when the backs were struck they would be offset compared to the front images. When being cut from the front you'd get misaligned backs.
Some were offset enough to show two and sometimes four parts of an add. These are from another column and/or row. 4 corners, 1 strike
The OM has two four corner strikes, one upside down and one upright.
Clear as mud.