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Page from a patent for the Robertson screw, 1909, with a diagram of the screw
Page from a patent for the Robertson screw, 1909
Source: P.L., inventor of the Robertson screw
by Ken Lamb -- Milton, Ont. : Milton Historical Society, 1998. -- vi, 186 p. : ill., ports. (2 folded) ; 24 cm. -- ISBN 0969562969 (pbk.). -- P. 23
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