Going back to Roman times (for the ancient collectors among us), and I would imagine that the story even appeals to the "love token" collectors as well. http://www.neatorama.com/2018/01/08/Its-a-Dog-Tags-Life/
Nice article. I got my dog tags when I enlisted in the Army in '67. Wore them for three years. Later when I entered college, one of our first tasks in a biology class was to type our blood. Mine kept turning out wrong - A positive. I knew that my blood type was O positive, because that was what was on my dog tags. Well, my tags were wrong, I'm A pos. I'm just glad I wasn't in a combat unit where I might have needed an emergency transfusion.
You would have been okay. O is the universal donor but if your type is O (or believed to be) then they could only give you that.
I still wear mine because they have allergy information on them. It's cheaper than buying a new medical alert bracelet.
Got my first set at Fort Knox in 1979. I was looking at some pictures and I’m pretty sure this is the model of machine they used to make mine all those years ago.
