Hold on to any 1964 and earlier silver coins - any kind - they have the highest silver content.
When you buy ‘junk silver’ it is usually a combination of those coins in a bucket. I have several that I’ve inherited from my dad. The value is based upon weight when you go to cash them in.
My local shop verifies the coins as being pre-1964, and pays me on the number of coins, not the weight, although that would make sense as many show substantial wear and are unlikely to be at full weight.