Mercury Dimes - major varities

Not my area of expertise. But I'll be helping a friend with some soon (some circulated rolls). Can somebody point me to where I can find information on the major varities in the series? Photos if possible...
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-YN Currently Collecting & Researching Colonial World Coins, Especially Spanish Coins, With a Great Interest in WWII Militaria.
My Ebay!
Some double die coins are available starting in the late 20's, 29 to be exact, and going through the 30's. They are tough to see.
Also you have a type of 16 coin for 1917 and a type of 17 coin also.
One last thing. In 1924 on the reverse a portion of the design disappeared. This is present on some coins but not all for this date. Same is true for 1925. In 1926 all of the coins lost this minor portion of the design and this continued through the end of the series.
Cool little things if you like this sort of thing.
Ken
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