It's not an "L", but rather a "Z". This strap is worn by those members of Ersatz Feldartillerie Regiment "Zossen". There were four such regiments, with this type of strap with the metal initial. The others were for Beverloo (B), Jüterbog (J) and Altengrabow (A).
These letters were authorized in June of 1915 and were originally to be made from brass. Later war examples were stamped from steel and painted feldgrau. Here is a M1915 example from the "Jüterbog" regiment.