More or less, I think, the riddle is solved. Below is a photo of a pickup cavity of a guitar, which had p 90s and mini hums installed (i forgot, which was in first - not important) the two inner screwholes were from the p 90s, the outer ones from the mini hums. As one can see on the picture "goldtop 2" in my earlier post, the guitar in question had at some time - we don´t know when - also both of these Pickups installed: the inner screwholes stem from the p 90´s. The outer holes are only halfway preserved, because they were cut through when the cavity was enlarged for the humuckers which the guitar uses now. The thin binding on the cutaway clearly shows that the guitar was originally one of these p-90 reissues of 1971/2 with wraparound tailpiece. It was originally delivered with p 90´s, then changed to mini-hums, and thirdly equipped with the t-tops it carries now. As many of You pointed out, ther must have been a refinish at some point. Thank You all once more for Your contributions!! Very competent forum here.