Used as an idler pulley. See http://dkprojects.net/belt_drive_brompton/.
The center of the pulley is designed as a press fit for a metric ball bearing, 22-mm O.D., 7 mm width. In one printing press fit worked perfectly, in another it was a little loose -- I added a brass shim.

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I bet this would be a great desk toy.

Steampunk Ring