1)The first bit is easy, as the first letter of each plate spells WHODUNNIT IX
(A challenge to our Inspector.)
2)The second bit is a little trickier, but I gave you the solution.
If you read the last three letters in each plate from the bottom up and right to left you to get
ADVANCE EACH LETTER BY THE NUMBER SHOWN, so advance W by 5 to get B,
H by 13 to get U and so on until you spell BUTLER DID IT.