Trying to date a pair of vintage Blue Bell, Inc. Dungarees

Normally I feel pretty confident in my ability to nail down a particular decade or era but I have a pair of dungarees that have a Blue Bell, Inc. label inside but no actual bell label and no Wrangler tags either. The snap closure is throwing me off and zipper is a Scovill.

I'm attaching a few pics here. Any help would be greatly appreciated. Thanks!

 

Blue Bells are tricky to date because they are a workwear item and the designs with them are classic but... I think with the washing instructions, you could say 70s/80s approx.