I got a $100 bill at work from an international traveler and it was super old but still crisp and new looking. It DID NOT have any sort of security strip or a watermark but the paper was legit (blue and red cotton fibers that are impossible to mimic). My boss was too scared to take it just in case it was fake so I had to keep it as part of my cash out for the night.

My ATM wouldnt even accept it as a deposit since it was the old style bill so I gave it to a cashier to break for me. He tested it with a pen and everything was good to go! I tried looking up these bill to find out where they were coming from and it believe it was from Europe, likely France, as part of some sort of USD cash reserves. They only started adding the security thread in 1990 and the watermarks in 1996...