The ‘duck and cover’ drills of 1950s America are infamous — mostly because the idea of a flip-top desk acting as some sort of impenetrable shield while the rest of the school is wiped out in a fiery mushroom cloud is pretty ridiculous.

But as it turns out, the sweet and slightly terrified-looking schoolchildren in those grainy photos, with their Brylcreem and bows, weren’t hiding from the A-bomb at all, but the adult brandishing a tattoo gun.

With most of the blood collected by the US Red Cross going overseas to help the Korean War effort, a nationwide blood-typing program was devised to allow the quick identification of donors in case of nuclear attack.

Schools across the Cold War-panicked country decided to imprint both the ABO blood group and Rh factor on the left side of children’s chests, essentially turning them into “walking blood banks” for the severely injured.