Flat belt pulley. It measures 25.643 mm dia with a 7.938 mm width. In O scale this is a 48-1/2" x 15" pulley. The rim has the correct crown for this size pulley. The shaft hole is 1.588 mm/0.0625" idia. The pulley was designed using data from "A treatise on belts and pulleys"
by John Howard Cromwell pub. 1888