Below is a missing-letter puzzle made by me. You need to find 4 missing letters inside the grid.
Can you find them?
Every row and column has some pattern.
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$M$ and $N$.
$S$ and $U$.
Because in each row and each column,
taking alternating letters (1st, 3rd, 5th and 2nd, 4th, 6th) gives two arithmetic progressions through the alphabet, assuming the alphabet wraps round on itself so that $A=1=27$, $B=2=28$, etc.