The 1776-1976-D Bicentennial Quarter is indeed a fascinating coin for collectors, and some varieties, such as the DDO (Doubled Die Obverse) and DDR (Doubled Die Reverse), can be quite valuable due to their rarity and the interest they generate among numismatists.
Doubled Die Obverse (DDO): This refers to a coin where the design elements on the obverse side (heads) appear to be doubled due to a misalignment during the minting process.
Doubled Die Reverse (DDR): Similarly, a Doubled Die Reverse occurs when the design elements on the reverse side (tails) of the coin are doubled due to misalignment during minting.
Filled IN D: "IN" refers to the designer's initials, Jack L. Ahr, which are located below the wreath on the reverse side of the Bicentennial Quarter.
These varieties can add significant value to a Bicentennial Quarter, especially if they are in good condition and well-preserved.
As with any collectible coin, it's essential to have them authenticated and graded by a reputable third-party grading service to verify their authenticity and assess their condition accurately.