If you're a person who believes it's a bogus exhibit, then you have to come to an acceptance that one or more prosecution witnesses had involvement, in some form, alongside any officials that were involved. I suppose you're right - it is speculation to try and pinpoint who and I tend to regard it as some kind of 'joint venture'.
I personally suspect DB has involvement. Unlike some others, he's vocal in the media, as if he's a stakeholder in the conviction (because of his previous actions).
Robert Boutflour mentions the silencer striking the mantle-shelf in his diary. This was long before Brain Elliot at huntingdon labratory confirmed this via microscopic examination.
Ann Eaton mentions what appeared to be coagulated blood on the silencer. Had that blood been there since the 7th of august, there is no way you can tell its blood by simply looking at it. It would appear like dark rust by that time.
In late August, Robert Boutflour tells Barlow that he thinks Jeremy used Sheila's tampons in the lounge to clean the silencer after the killings. (tampons are designed to absorb blood and are shaped like a tube). Thus he is insinuating at there is or was blood inside the silencer at some point. This was before the silencer was dismantled and blood was found inside.
They know too much, and know before even the folks at the lab know it. The only logical explanation is they created it in the first place.
