Ok I think I found it reading through all the Baron 58 project. In the main panel section theres a small excerpt about the lettering:
The letters are simply printed on self-adhesive thin paper, with very carefully chosen color of lettering background to fit the panel color, and the paper is cut closest to the letters. It's not a "long living" lettering, though it looks good enough and easy to repair.
Also in the annunciator panel section one can see how they use this trasparent label.
I guess this was used as well to do the printed shapes in the faceplates of some instruments, like the audio panel (those rectangular outlines sorrounding groups of buttons):
I Wonder if its the same lettering that was used for buttons, as lettering in the panel is not normally messed with, while lettering in the buttons is constantly put to test as we use the sim and tamper constantly with the buttons, so it can become very short lived?