No need for a smiley attack.
This is how the sheets were cut up for booklet panes:

Positions W5, W6, W11 and W12 have stamps with a narrow bottom margin. W5 and W6 show the horizontal sheet cutting line (which is much closer to the design than on the regular sheets - this is a dead givaways for faked AEF panes made from ordinary sheets).
Your stamp could be a bottom stamp from positions W11 or W12. Only a certificate will tell, there are minute differences between ordinar sheet stamps and booklet stamps but this requires examples for a comparison.