A listing of "1937-D" is a coin minted in Denver, they used a "D" as a mark.

An "S" is the mintmark used by the San Francisco mint. These two mints placed their mintmark on the reverse below the buffalo. The third mint to strike Buffalo nickels, Philadelphia, did not use a mintmark.  This coin shown up for auction has no mint mark.

A very distinctive mint made error once you have seen it, the 3 Legged Buffalo variety is popular with collectors and considered necessary to complete a collection. These are two powerful forces driving the value of the 1937-D 3 Leg Buffalo nickel.

The 3 Legged variety is found on nickels minted at the Denver mint. Locate the "D" mintmark, identifying the mint, below the buffalo under the "Five Cents."

Excessive die polishing is believed as the reason too much metal was removed causing the buffalo's leg to vanish. Before the mint realized the error a few thousand had been released into circulation.

Back in the day many of these now valuable nickels circulated freely, the result is 3 legged buffalo nickels are found today showing all degrees of wear. They can turn up in any box of old coins.