Sorry, but the probability of finding genuine fatman dollars at a flea market is very, very low, compared to the probability of finding fakes, which until recently were being shipped into the country by the shipping-container-load. It is certain that these are fakes, and extremely unlikely that they will be made of silver. Whatever "test" you are using for silver, it's giving a false positive with these coins.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis