Why are so many dimes missing a mint mark?

With a three-year exception from 1965 - 19667 when no mint marks were used at all in spite of circulating coinage being made at Philadelphia and Denver, all coins with no mint mark are from Philadelphia. One cent coins to this day do not use the P mint mark. Beginning with 1979, the P appeared on the Anthony dollar, and in 1980 was added to nickels, dimes, quarters and half dollars. It has been used on these coins ever since, but is not and never has been used on cents.

The only other time prior to 1979 that the P was used was on the 35% silver 'war' nickels dated 1942 - 1945.

So, any dimes from 1968 to 1979, and before 1965, that have no mint mark, are from Philadelphia. 1965 to 1967 are from either Philadelphia or Denver, or if it's in one of the Special Mint Sets issued these three years, it's from San Francisco.

Because for most coins there is one primary mint that puts no mint mark on the coins - often it is the Philadelphia mint in the coins when I was growing up. Then a mint mark is put on smaller mints that also produce the coins such as Denver and San Francisco.

Apparently, I was dealing with WWII vintage coins (which is when I was growing up) and this article explains what has changed with history http://www.usmint.gov/about_the_mint/collectors_co...