Here is a US/International Maritime Signal Flag. The flag is a blue and white three striped flag. The signal flag is "Juliette" which in both International and US Codes means "I am on fire and have dangerous cargo on board, keep well clear of me" or "I am leaking dangerous cargo".
I pin it around Vietnam/Late Cold War because the material it's made out of is indicative of materials that would've been used at the time.
The flag is marked "R"
Shipped with USPS First Class