What US mint did not mark its coins in the 19th and early 20th centuries?

In researching the value of some inherited coins, I find that many listed for sale on the internet have specific mints marked, like "D" for Denver, "P" for Philadelphia, or "O" for New Orleans. Many of my coins don't have a mint mark. I wonder if this also affects the coins values?

What US mint did not mark its coins in the 19th and early 20th centuries?
With a few exceptions, coins minted at the Philadelphia Mint, the nation's primary mint, did not bear a mintmark. (One notable exception: part-silver Jefferson nickels minted during World War II, which had a large "P" over the Monticello dome.)





Occasionally, however, in some listings of coins for sale, some Philadelphia Mint coins may nontheless be designated with "P" or "-P" following their dates, to fully identify these coins as originating at the Philadelphia Mint, and thus resolve any possible ambiguity.





Coins have value based on their value to collectors, and in some cases, on their metal content (e.g. for silver coins that might otherwise have little collectable value). Some Philadelphia mint coins are scarce and valuable, while others are common and have little value; it just depends on the denomination, series, and date of the coin, and its current value to collectors.





You can find many coin value guides on the Internet. Some of these include:





http://values.hobbizine.com/coin-index.h...


(Retail prices, but fairly realistic)





http://coins.about.com/od/coinvalues/Coi...


(Very realistic prices)





And for earlier coins:





http://www.pcgs.com/prices/


(Fairly high retail prices, but a good general guide nonetheless)





As for grading (condition), which is a major determinant of a coin's value, you might start here:





http://www.coin-gallery.com/cggradingart...





There are also grading books that you can buy, or find at a library or bookstore, that offer detailed photos or line drawings that you can use to more accurately assign grades to your coins.
Reply:Yeah, it's the Philadelphia mint. Pennies are still made without the P mintmark.