Can't tell for sure from the close up view you posted, BUT, my guess is it reminds me of a part from a wood workers bench called a "dog".
was placed into square hole in the top of the bench to provide a back stop for a piece being worked on the bench top. You would open the bench vice in front of the bench, place your work piece [a piece of wood to be drilled or planed with a hand held wood plane] and then put the "dog" behind the work piece. You then raise a small metal tab on the front face piece of the vice, and tighten the vice squeezing you're work piece between the "dog" and the vice face. The top of the work bench usually had several of the square holes spaced out directly in front of the vice to accommodate various lengths of work pieces.