There's 3 different steel springs. Ask me how I know! I'm repainting one more hinge of mine now because I put a 73 with a 70 and my hood opened funny.
Here's how you tell.....
1970 is a one year only. It has a narrower top bar that the hood mounts to. It also has teeth near the pivot point.
71 - 72 are very similar to the 70, but have a wide top bar where the hood mounts and larger pin rivit on the rear top strap.
73-74 have a knee action and have no teeth.
T/A and AAR are springs with 35 coils.
Hood hinges have a four digit date code too!