This is the key that mounts in the crank on the left side of the engine to guide the flywheel into the correct position.
Note.
This is purely a locating guide it doesn't stop the flywheel from
turning. The taper on the crank holds the flywheel in place so the
flywheel nut MUST be tightened to the correct torque, 50-54 lbs/ft or
the key will shear.