What I find interesting, are the contents of a handwritten note sent by "Ron" Cook (SOCO) to the ballistic expert, Malcom Fletcher, about the silencer being possibly damaged due to it having received a hard knock, and that when test fired bullets were being fired through it, they were becoming damaged. Cook wanted to know if the silencer could be warped after striking a hard surface?
What this tells us is that before the silencer was sent to the lab' police had conducted tests by firing control bullets through the silencer, and that these control bullets were fragmenting badly. This is very interesting, since if the shot under Sheila's chin was inflicted with the silencer attached to the barrel of rifle "Y", the bullet in question should have fragmented as it passed through the 17 misaligned baffle plates so that when it ended up inside Sheila's brain, it could not be a whole bullet? On the other hand, it leads one to suspect that in the case of the first shot (PV/20), the one which was fired slightly off centre but to the left of the throat, that the damaged silencer could have been used. Indeed if you include the corresponding ring of bruising around the lower so called non fatal entry wound site, it could be an indication that such a silencer which (during unreported test firing using control ammunition) was causing bullets fired through it to fragment, may have been fitted to the barrel of the gun which fired that (PV/20) bullet?
This would explain why police or somebody connected with the investigation, decided it was best to substitute or swap over the original fragmented bullet (PV/20) for a test fired control bullet so that the argument could be put that both bullets recovered from Sheila's neck were whole, and easily linked to the rifle (Y) because both of those bullets had been fired via rifle "Y" albeit at different times, one (PV/19) fired via rifle "Y" during the shootings, and the substituted or replaced bullet (PV/20) fired via rifle "Y" later on during these unreported test firing...