Plus, think of how difficult it is to match a person to the physical envelope.
At best there could be a distinctive envelope.
Otherwise, yes, you can get a list of people who use the box. But for that to be useful, the mail from different boxes can't simply be jumbled together into the same pickup bag as that would broaden the number of suspects.
Even then, here is literally the first post box I found looking in the UK, in a small town: https://www.google.com/maps/@52.0936599,0.0761217,3a,75y,165... . No CCT in sight, no power, good solid iron.
Plus, think of how difficult it is to match a person to the physical envelope.
At best there could be a distinctive envelope.
Otherwise, yes, you can get a list of people who use the box. But for that to be useful, the mail from different boxes can't simply be jumbled together into the same pickup bag as that would broaden the number of suspects.