Help With Silver Planchet Error

Breens mentions 1971-S Proofs only may exist on leftover silver-clad blank. Same comment about later strikings.

thanks biokemist....yes I believe you are correct about rarity. but then again, I've seen hundreds of thousands of Kennedy half dollars and if there is a chance I might identify something good, I try to educate myself on it. I never thought I would find some of the great errors I now have in my collection just by looking hard. A very sucessful error coin dealer told me recently that when he worked with counting rooms to get error coins, he was surprised to see just how many there were. He cited an example of off planchet coins. Most would say that these are quite a rarity, but he said that he would get new bags of coins and there would be quite a few in one bag. There is so much to learn and so little time. thanks for your help. In regard to the weight, I'm not sure if that clad error for 1971 halves was 40% or not. Of course the silver content would affect the weight, but I could spot a 40% silver 1971 kennedy from half a mile by sound of one clinking on another and of course by appearance. I think the planchet in question had a lower silver content which could partially account for why so few are located even if they are out there. These would be harder to find by sight or sound.