The German Field Artillery used the 0-6400 mil scale until the end of WW1 (1918) and continued its use until the current day.  The German Foot Artillery used the 0-5760 scale which is 1/16 of a degree (360) and an expressed version of this scale 0-5800 until 1916.  Conversion between the 6400 mil scale and the 5760 scale was required if a forward observer called for fire for both Field and Foot Artillery firing elements. Otherwise, the forward observer would have to be equipped with both 6400 scale optics and 5760 scale optics.  After 1916, it may also have been used as a way to maintain common survey with Foot Artillery firing elements that had not yet received updated 6400 mil scale instruments.  The conversion scale example I have is marked to an Ersatz Foot Artillery Regiment.  It seems possible that Reserve, Landwehr, Landstorm, and Ersatz Reserve Foot Artillery equipped units may have been slower to convert to the universal 0-6400 mil scale and this paper conversion table was a cost effective way to work around the problem 

The innermost scale in black ink is the 0-5760 scale.  The middle scale in black ink is made of two 0-3200 scales equaling 6400, and the outermost scale in red ink is the 0-6400 mil scale. A center mounted string held to the known number, such as 2980 mils then also indicates the 0-3200 and 0-5760 scale equivalents.   

Formulas for conversion between the Field Artillery and Foot Artillery scales are indicated on this chart by quadrant.  Depending on which quadrant of the Field Artillery 0-6400 mil scale or the quadrant of the 0-5760 Foot Artillery scale the number to be converted falls within, a different formula is used