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It's a "Spline Weight", used by a marine draftsman.Here's an explanation regarding an available new model. http://westlawn.edu/news/index.asp?displayfile=edsonsplinewgtsweb.htmA K&E version.http://www.ebay.co.uk/itm/Vintage-K-E-KEUFFEL-ESSER-NOS-Spline-Weight-Drawing-Drafting-Tool-/201137183425?nma=true&si=GwMr6mV8ssbRYB56X2pKUs8jq3o%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557