That's news to me, although 1964 was the last year silver was used in dimes and quarters. But silver still circulated for a few years after. Stamping a code on a silver dollar would hardly keep it from wandering outside in someone's pocket, nor would it reduce the melt value.

Even a few years ago - before TITOs - I was getting an occasional silver half dollar from slot machines in Indian casinos in Michigan (Indian casinos were still using coins rather than tokens, at least for amounts under a dollar).