What is this?
Never seen this before on any bike I have owned. My first guess is a chain holder during wheel removalImage: https://cimg9.ibsrv.net/gimg/bikeforums.net-vbulletin/2000x1504/dd7717e9_2fac_4c88_a075_cce5b8e9f79c_fc65792e2cee33cb2f7a1f9f39298d79cef6473f.jpeg