How thimbles get out of round.

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Used on tape printer positions to cut off tape when gumming it down. A round thimble-shaped piece of nickel silver set on a flat polished plate, the edge of which is used as the cutter. Furnished in sizes #5 to 12 inclusive, thimbles being of different diameters and lengths. Plate 3/4" x 1 1/2", Ht overall -- 1".
[Excerpt from: The Western Union Telegraph Company Inside Plant Catalog of Apparatus, Material and Tools, July 1, 1929]