No but out of interest sake what is the law in USA & does it differ from state to state?

It differs from state to state and sometimes even city to city. New York used to require just a dog license on the tag, but there are no requirements for what must be listed on a regular dog tag or even if your dog has to wear one. Now they've stopped that and it's up to each city to come up with licensing rules (which who knows what those will be).

I'm not sure outside of New York what the laws are, if any.