While sorting old bricks I observed that the inscriptions printed inside show a different coding. As you can see the outside it is always a ordinary "1x2 tile with groove in good old Light Gray (item number 3069)". The inside instead appears very different.
Reading from left to right it says:
15; (c) LEGO; 032 # LEGO; 3 # (c) LEGO; 16; 35; 3069 # 8; (c) LEGO; 032 # LEGO; 1 # 3069; 36; 17; (c) LEGO # 5; LEGO; 3; 1 # LEGO; 11; pat. pend. # LEGO; 11 # 3069; 36; 9; (c) LEGO
Why is that so?
You can also see that most of the bricks got on one half of the inside a yellowing. It seems that the effect is not only accruing on the surface but really eating its destruction through the material which I was not aware of.