Varieties by German State 1900-1918
This list covers the 2, 3 and 5 Mark coins

Baden - Mint Mark: G
1902 to 1907 5Mark coin
I've not seen enough of these to know what the varieties are

Bavaria - Mint Mark: D
2Mark 1901 to 1908 and then 1912 to 1913. The variety is the open and closed hair curl on the top of Otto's head. It is a very noticeable curl.
The 3Mark coin 1908 to 1918 has the same variety as the 2Mark coin.
The 5Mark coin 1901 to 1904 and then 1906 to 1908. Minted again in 1913. The same variety as the 2Mark and 3Mark coins.

Burnswick-Wolfenbuttel - Mint Mark: A
3Mark 1915. Some of the coins have U LUNEB added to the obverse. The coins without the U LUNEB added are the most sought after, with only 1,700 being minted.
The 5Mark coin has the same variety as the 3Mark coin, with only 1,400 being minted without the U LUNEB added.

Wurttemberg - Mint Mark: F
3Mark commemorative (25th Wedding Anniversary) 1911. The letter H on the obverse in the name Charlotte, is known to have a Low Bar H and a High Bar H. The High Bar H is the most sought after, with a mintage of 7,000.
Its is very easy to tell the difference between a LOW BAR H and the High Bar H. If the H looks normal with the cross bar in the middle,..than it is a Low Bar. If the cross bar looks to be higher up and not in the middle, than it is a High Bar.

These are the only varieties that I know of during the time period covering 1900 to 1918.