These small metal clips plug into the holes in the hood, the nozzles screw into these which in turn also spread and lock. Make sure you screws aren't too long either or you'll end up with a dimple in your hood.
Also, if sourcing new or used make sure you get the flat style nozzles for 77-81 firebirds, the typical one sold which bends down is for 70-76 Firebird and all Camaro, as they shoot through the hood wiper grille, not the far edge, and would foul the cowl if you tried to close the hood!