It all depends on where the mark is placed on the back, and it's orientation (right side up, upside down, at 90 degrees, etc..)
Actually, only a few expertizers used such a convention, and it not only depends on the expertizer/group, but also WHEN they started using the convention. BPP is the largest expertizer group that I know of that utilizes a type of "position" convention.
Regardless, the expertizer would mark the stamp with "FALSCH", not just rely on positioning/orientation of expertizer mark. Most of the time, the purpose of positioning is in regards to the condition/type of the stamp (i.e., mint, used, non-overprinted, overprinted...). This was to circumvent altering the stamp after the expertizer mark was added (e.g., prevent someone from adding a fake overprint or fake cancel).
In the pictures above, all the A. ROIG marks are fake, just like the stamps.
Spain_1850, it would be great of you if you could post details about what to look for. I only used a single test for the 1851 issues: in REALES/CUARTOS, the AL/AR are connected at the base.
Thanks in advance!