Your photo with the arrows is EXCELLENT....
...Xose, I don't see a black plastic strip in that photograph. I can clearly see the pickup plug, the nut securing the bridge to the trapeze and the screw securing the pickup arm to the unused string slot in the bridge. The black area you see is the unfinished portion of the guitar top which was under the pickguard. The double arrow I drew shows you what I mean...
See the black plastic strip at Macca's photo and another one (=although this time a slightly later model, that is a bit shorter than the old one, hence the 'arms' to be secured to the bridge) I own. My Rosetti also had that plastic strip, in this case is white. All Rosetti/Egmond guitars with that system (=branded Royal PP-2) had that 'supporting device', either white or black (=I'm including some examples here...)
This plastic strip was the 'supporting device' where all the pickup/pickguard assembly was glued to, and it had on the neck side an assembly with two metal arms to be screwed to the neck. McCartney's black plastic strip became unglued after hard use in Hamburg (=or perhaps he himself unglued it...)
That type of pickup also had an arm which could mount the pickup to the fretboard. I think Paul bent the arm and secured it to the bridge using the trapeze nuts.
The picture below is of a vintage DeArmond pickup with a single arm mount...
Yes, but probably not A DeArmond, but a DeArmond TYPE, as the one on Macca's guitar is bigger, do not have the same shape (=DeArmond is 'sharper') and it has a bracket to secure it to the trapeze. See: