It’s the Carte Jaune (Yellow Card) which has a function of being a record of immunisations that is sometimes useful when travelling to or from regions with specific disease outbreaks (yellow fever, measles, polio etc). The US Government printing office may still print a version of this. https://en.m.wikipedia.org/wiki/Carte_Jaune

Edit: rogue capitalisation

I also have one of these still. They're still pretty common I think. I also think my dog has one that look similar :)

I told a doctor I'd received a few vaccinations but forgotten to bring the card, and they just added them with no other questions. I'm pretty sure that was true, but it's definitely not a very bulletproof set of credentials.