Mike A Schwab wrote:

Those are the metric equivalent of the gap
inside the chains. The outside width varies
by 6, 7, 8, 9, 10, 10N, 10ND, 11 gears.

Ah, seems to check out. The 1/2x1/8" notation
is (plate length) x (roller width) with "roller
width" = "gap inside the chains".

$ units '1|8 in' mm
* 3.175
/ 0.31496063
$ units '3|32 in' mm
* 2.38125
/ 0.41994751

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