I have a line engraved a few mm from the end of the swing arm, 4 mm on the left and 3mm on the right to be exact. Do all swing arms have lines engraved for alignment?

If i line the chain adjuster up with the very end of the swing arm im 1 mm out as far as this alleged engraved line.

Ive always aligned the rear with the line and the chain adjuster, and now im thinking maybe some one has just engraved these lines for whatever reason. i think i should be using the end of the swingarm and chain adjuster.
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