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JJ109 got in first, but thanks also to bobbinwales & to emmie.
Breaking into the puzzle is a bit tricky as there are six suitable 4-letter grid positions. These divide into three that end with an 8 and three that start with 19. As 8 is very frequent, it's unlikely to be K whereas 19 is infrequent and could reasonably be F.
From there on it's a matter of trying out various combinations, but 12 is a doubled letter, which helps find the correct solution.