I misunderstood you when you said Darwin haha, no offense taken! You can probably see why this had me scratching my head when I went to put the window back in.
It seems they just etched the glass on either side, willy nilly! Makes me wonder how they manufactured it.... Did they form it to shape first, then etch? Or, was it etched on a flat piece and then formed? It seems if they cut the shape of the window out on a flat piece first, they could make left and right windows from the same template depending on which way it was formed. That would explain the etch being backwards sometimes.
My car is a 72 by the way. If I'm decoding the glass correctly, the 26-42 means PPG plant #26, made in April of 1972. I noticed the passenger side windows and rear glass are dated 4/72 while the driver side (the side that reads from the outside) is dated 5/72. It's quite possible its been changed out before.