There are no mint marks on US coins 1965 1966 and 1967. The 1939 isn't even worth a dime because it is destroyed and a common coin with a high mintage. It's just old.
1938D & S, 1939D & S and 1950D are considered the keys to this series. There are also doubled die errors in this series but best to check for them at www.doubleddie.com. Perhaps someone else can link you to over dates, OMM's and RPM's.
I'm sorry, I did not see you were a new member. OMM=Over Mint Mark (a Mint Mark over the first Mint Mark. And RPM=Re-punched Mint Mark, the two are completely different in how they were made. www.briansvarietycoins has a lot of these along with photos.
@siqueiros I agree with the other posts, however, if you are a new collector, you may want to hang onto the 1939. As said, it's not worth much but it's unusual to find it in change. One suggestion is to try to save as many different dates and mintmarks from the Jefferson Nickel series that started in 1938. You already have a on old one to start. If I've managed to pique your interest, go to a book store and look at A Guidebook to United States Coins. It will list all of the dates, mintmarks and a whole lot of other info. If you're interested in coins, this will be the best $10+/- you'll ever spend. They also have older editions at most libraries
If you are talking about nickels 1950 D isn't a key date. What you need to look for are full steps on nickels. That's what makes the nickels valuable. You will see on Ebay a lot of people claiming full steps but they aren't for they have breaks in the steps. Be careful when buying any coin. Do your research so that you know what you are buying. To see what nickles to look for look on PCGS or NGC and see what values are for any date. They will have 2 values MS-Mint State or Full Steps for all nickels.
