The "M" in the right corner of the coin stands for "Mint". It indicates that the coin was minted in Philadelphia, the only mint that produced Standing Liberty quarters during its entire production run.
Additional Information:
- The Standing Liberty quarter dollar was designed by sculptor Hermon Atkins MacNeil.
- It replaced the Barber quarter dollar, which had been produced from 1892 to 1915.
- In 1931, the Standing Liberty quarter dollar was replaced by the Washington quarter, which is still being minted today.
- The Standing Liberty quarter dollar was minted in 90% silver until 1929 when it was switched to a clad composition. The "D" mint mark was used on the Denver minted issues of the quarter from its introduction until 1928.