You are correct Mike. I also suspect you are right about what the missing negatives show.
yet to be fathomed out, is who had possession of each of these two different camera's and negative strips? This is important, because of the camera which has the two pictures in colour, must have been in the possession of PC Bird, and i am guessing the other camera which took the black and white image was probably in the possession of the training team or someone who was overseeing what they did or were doing?
Now...
if true, and because of the photographs I have seen of Sheila on the bed, it leads me to conclude that the training officers could have been responsible for Sheila being shot for the second time, hence why there appears to have been some sort of a cover up about the role they played and who formed part of this training exercise?