Straightforward, quite elegant, and no ambiguities in the preamble. In fact very little preamble. So, for once I'm not critical of Kea. I'm surprised solvers weren't asked to enter the theme below the puzzle, though probably the theme must first be deduced in order to resolve the unchecked cells in 15 & 23 : which I have as M and K respectively.
M at the end of 15, not N.
I have BRUT for 4, but didn't understand the wordplay.