Type A is the normal reverse for the 1956-1964 business strike quarters. Type B is the proof reverse. The Mint (probably for economic reasons) used the dies intended for proof coins struck business strike coins with the proof reverse dies. The easiest way to spot these coins is the separation between the bottom of the E & S in the word STATES. The Type C reverse is the reverse of 1965 and is only found on 1964D pieces. These coins can be identified by the split tail feathers. You can look at a 1965-1997 clad quarter and see the C reverse and the difference in the tail feathers. If you find a 1964D with the Reverse C it's worth at least $50 in VF grade and over $300 in MS65 grade. Check those silver quarters!!