It is a horse harness "bearing-rein swivel" invented and patented in 1880 by a Mr. Dowlin, US Patent #223,896. See the diagram in the 1907 Pierson-&-Hough horsegear mail-order catalog, shown in the attached scan, below.
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horsegear_gag-chain and martingale_Pierson&Hough-1907-catalog_1_TN_photobyCreskol.jpg