A 1976-D Bicentennial Quarter struck on a 40% silver planchet is a highly rare and valuable error coin.
During the United States Bicentennial celebration, quarters were minted with a special design to commemorate the event.
The standard composition for these quarters was 91.67% copper and 8.33% nickel, with no silver content.
In this case, a 40% silver planchet intended for Eisenhower dollars or Kennedy half dollars might have been fed into the quarter press.
The value of such an error coin can vary widely depending on its condition and the demand among collectors at the time of sale, but it can command a significant premium over a regular Bicentennial Quarter.
It's advisable to have such a coin authenticated by a reputable coin grading service to ensure its authenticity and maximize its value.