This motor arm fits a Brite Star replacement motor aka "deer motor," often used to make Halloween props and other animated holiday decorations. One side fits the reindeer motor shaft well and has a hole for the set screw that comes with the motor, the other side fits an 8-32 bolt. There are insets to glue in two 8-32 washers so you don't have a washer rubbing on the plastic - you'll have to put supports in the one facing the print bed.
The author hasn't provided the model origin yet.