What are these for?

Two (2) keys are gently tapped into the two (2) slots found in each corner, some professionals will use a small carpenter's "finish hammer" for the job. Other experts will place a small block of wood on the outside edge of the tightening key then gently tap the keys into place. Equal pressure should be used with each key into each slot to prevent future warping of the stretcher bars. Upon insertion of the keys, care is taken not to apply too much pressure to the keys so as to make the canvases too tight, possibly causing the canvas to rip or tear.

Never see those before. None on the canvases I've gotten.