The most common alteration: a D or S glued, soldered, tooled, or embossed onto a Philadelphia coin to fake a key date. Six loupe diagnostics: tool marks in the surrounding field, wrong font/punch style, wrong position vs. genuine examples, seams/edges at the letter's base, discoloration or texture mismatch, the "too perfect" mint mark on a worn coin. The 1916-D Mercury dime is the textbook case — and Jacob's set still needs the real one.
Evidence standardThis reference is distilled from a covered source topic. A photograph can inform a decision, but it cannot authenticate or certify a coin.