Yes, Proto was assimilated by the Borg, er... I mean Stanley-Black & Decker.
Proto has a sub-brand called Blackhawk. As far as I can tell, Proto is still primarily US manufactured while Blackhawk is mostly made in Taiwan.
From Proto Industrial Tool's website:
"WE ARE MADE IN AMERICA.
We make tools in America; Not because it’s cheaper and not because it’s convenient. We don’t even do it out of a sense of pride (although we are proud of the U.S.A.). We make tools in America, because put simply...it’s how we can ensure making the best quality tool we can. And in the rare instance when we can’t make a tool in America, we do so because... again, making the highest quality tools comes first. "
I've found that the customer service hotline reps at companies can usually tell you if a specific tool you're looking at is US made.