the 71' and 77' are 40% silver because over those years when they discontinued using 40% silver slugs for coins They still had and stored part silver slugs well in 71' and 77' they had a mishap mix-up and used 40% silver slugs insted of clad slugs in the years of 71' and 77' feel free to look it up on the internet and check it out and you can tell buy looking at the side of the coin and you will see the difference of clad and silver and if you put those two years in a large roll of halves and look at the sides of the coins you can pull out those 2 years everytime next time you get a roll try it once in a while a 72' will do the same thing I have always wondered if 72' got some mixed up also