The photo in your post is not of a police officer in the case and does not show the murder weapon. It is from an article in a French newspaper.
It was originally in the English M/C Murder Casebook series no.7, as in the photo below I've just copied, then probably featured in a French translated version. It's one of series originally taken by the APA at the time of the murders, like the one of an officer/detective retrieving evidence from Nevill's car outside Bourtree cottage.
The white-painted woodwork is reminiscent of that at WHF, maybe of a store room/walk-in cupboard? Or the small Cup'd in the hallway adjacent to the Bottle Cupboard?
If he isn't a police officer, have you any idea who it might be?