I'd have to look up the date, but the point is they're still valid currency (though the silver is worth more than a dollar). His mistake was making something that looked like real US currency. All he had to do was make something that looked unique and he would have been fine.
There are also current issue US bullion coins that are based on other earlier designs, not to mention all the commemorative one-offs they come out with all the time, some of which are pretty amateurish looking IMO. It really isn't a big stretch to say that some people would mix them up with gov issued currency. On top of that the "Trust in God" paraphrase is just asking for it.