Originally Posted by
revcp
There does indeed appear to have been an "old" 510 that came in square taper, but the 510 on my Surly Troll is Hollowtech.
Good point. Looks like Shimano offered both versions within same model generation (see photo for tech specs).
I believe Shimano does the same thing with offering both a square taper and Octalink option within some Altus/Acera/Alivio generations.