Here's a couple explanations of the difference in ebody hood hinges taken from other forums.
Yes, there are 3 or 4 different hood hinges. In 1970 they used a smaller bolt (I think it was a 5/16") on the hood to hood hinge and a smaller "top bar" that bolted the hood hinge to the hood. Those also had the "gears" or teeth in the hood hinge. If I recall correctly, in 1971 they went to a larger "top bar" and a larger bolt, I think it was a 3/8". They kept the gear or teeth style in 1971. Later on (I think around 1972) they came up with a non gear (teeth) style hood hinge still with the larger top bar and 3/8" bolt. Those are the 3 that I can think of right now.
What's the visual difference?
The width of the slot
1970 - clearance for 5/16" bolts, ~3/8" wide
1971 - clearance for 3/8" bolts, ~7/16" wide
To accommodate the larger bolts used.