What do you wanna bet that when and if there’s a trial, the knife in question is either ‘lost’ or is replace by a ‘legal’ one...........
To introduce any physical evidence the prosecution has to establish a chain of custody and most objects are marked in some way. So, to switch knives, one or more cops would have to commit perjury.
From what we've seen and heard so far, I'd be surprised if the evidence didn't include a can opener and cub scout badge.