The subject of signage for all the different types of Boxes in the UK is a varied and interesting one. Everyone seems to have had their own idea of what was appropriate.
It should be noted that the Met Boxes were originally designed by Trench to be completely without such signage; the only signs being the top, perforated stencil type"POLICE" signs that doubled as the Box's only form of ventilation, and an instruction sign on the inner door of the phone cupboard that faced outward so a user knew the proper procedures for use of the telephone, like Sillitoe's Sheffield plaque shown above. You can see a little sliver of the Met instruction sign in the above photo - in fact, it almost looks like the woman is reading it while she's on the phone.
Met memos show that by early 1930 the lack of signage explaining that the public could have full use of the phone for purposes relevant to the Police was proving to be a problem, and the top signs were all but invisible in the dark unless seen at just the right angle, so steps were taken to rectify both issues. The first approved attempt at this would have had a needlessly verbose sign immediately opposite the phone door with a giant "hand" above this pointing at the telephone itself, similar to the arrangement on many Police Boxes in other cities at that time. Fortunately, this was altered (I suspect by Trench himself) to the more elegant final design on the panel immediately below the Phone Door, these being installed by mid-1930 on all Boxes.
The above photo appears to have been part of a publicity campaign later in the year (or early 1931) to advertise the new signage and to promote public use of the Boxes. I'm pretty sure the Box pictured is V11 opposite the Gasworks Bridge in North Sheen.
Here's a slightly enhanced blowup of that sign:
Interestingly, some sources seem to suggest that this sign was actually "in glass" like the later integral Phone Door signs rather than an "enamel plate" like the St. John badge/plaque.
BTW, both Mark 1's and Mark 2's had that high lock position (and a different model and/or make of lock than the ETAS found on the Mark 3s and later). Also, photos of the Mark 1's seem to show that though the small knob on the Phone Door was always supposed to be there, that big knob on the main door didn't show up until some time in 1930, so was not part of the original design. I would guess that over the winter it was found that the wood had shifted and settled on some of the Boxes, making it difficult to open the door using just the key to pull with. Both these knobs were fixed, ie, non-rotating handles, and though many were replaced at some point with the Mark 2 and later standard bow handles, some were not. The St. John signs were always supposed to be attached, but they weren't actually installed until the Boxes were finally equipped with the St. John First Aid boxes, by circa April 1930 (the delay being due to logistical issues).
