These trucks didn't have a spring in the latch area to pop the hood open. People may have added one if their hood hinges and/or hood bumpers were worn out.
The hood hinge springs and rubber hood bumpers (if they're adjusted properly and not worn out) are what pops the hood up a bit when you pull the hood release lever in the grill. I've read where some people have to rest their hand on the hood to prevent the hood from flying open when pulling the lever in the grill. I guess a lot depends on lubrication and the strength of the springs on your hood hinges.
There are a few threads discussing this issue and confirming these truck did not come with a latch spring from the factory. Here's a good place to start
https://67-72chevytrucks.com/vboard/...d.php?t=736475. Looks like the issue can be resolved by adjusting, lubing or replacing worn parts.
There was a member who produced and sold a retrofit hood latch spring but I can't remember the user's name.
EDIT: Here is the thread by the member who offered an aftermarket "hood popper"
https://67-72chevytrucks.com/vboard/...ht=hood+spring. Looks like he's not selling them right now and trying to develop a Gen II of the product.