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So I measured...
They are actually different widths. My vernier calipers have run out of battery but using my extremely precise 'ruler' method (very much not Tester approved), the 11 speed one is a shade under half a millimetre thinner at the base of the tooth 'trough'.
Anyway, very encouraging to know that the internal widths are the same, and what with the important new discovery of this Zephyr chainring in my parts bin, I am now at liberty to just try it out in real life. If it works it works, if it doesn't I'll get something that says 11 speed on the box and the Zephyr can go back to waiting for my other bike to need a new chainring.
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My vernier calipers have run out of battery
If it is battery powered, then it is probably not a Vernier caliper 🙂
Don't just look, measure. 9/10/11-speed chain is all 11/128" roller width.