SOLUTION:

The culprit is the BUTLER.

The first letters of the plates in the given sequence read ‘WHO DUNNIT‘. This gives indication that it is an encoded message to the detective.
The last three letters of the plates read from bottom right (last plate), from right to left, upwards gives the clue as “ADVANCE EACH LETTER BY THE NUMBER SHOWN’. Accordingly, the first letters advanced by corresponding number in a circular fashion (i.e., A to Z and then again starting from A) forms the clue to the killer. (e.g. W 05 means W advanced by 5 places which results in ‘B’ (as X, Y, Z, A, B). Similarly, H 13 gives 8 + 13 = 21 => U, the 21st letter, O 05 gives 15 + 5 = 20 => T, and so on, so that the first letters of all plates decoded gives ‘BUTLER DID IT’.

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