Experts recognize two different 1971-D Doubled Die Obverse Half Dollars as significant. They are slightly different in the way the die is doubled and where the doubling show up. On the FS-101, the doubling is most evident on the word TRUST, especially on the U, S, and T. On FS-102, the doubling is most evident on the I of IN.
The FS-101 1971-D 50C Doubled Die Obverse is somewhat scarce. Because the coins can still be found in Mint Sets, many of the known examples are in high grade. For example, all of the coins in the PCGS CoinFacts Conditon Census grade at least MS65 and go as high as MS66.