An insert to join two vertical sections of a boltless shelving unit.
There are two STLs -- one of the full piece, and one with it split into two parts to make printing easier. The two parts can be joined with acetone (if printed in ABS) or glued together.
See https://www.chrisfinke.com/2020/11/23/todays-functional-print-adjustable-shelving-leg-inserts/ for more pictures and details.