This is a stabilizing strut, for attaching the Vélo Orange™ Constructeur Rack on the rear of a bicycle at the seatstay-arm bosses. It is sized for a medium-size Vélo Orange™ Pass Hunter frame (2nd generation, circa 2020).
Two struts are required. Bolt the rack to the frame, just above the rear hub. Then bolt one strut to each arm of the seatstay and zip-tie the other end of each strut to the rack.
The author marked this model as their own original creation.