Hello Mark,
In the fifties and early sixties, Mercedes cars had this special 14mm allen tool included in their tool kit which came with the car. In later years this special wrench was available to purchase separately or was included in an optional auxilary lubrication tool kit. The 14mm allen Dowidat tool in this kit was labled Mercedes-Benz and displayed the Mercedes part number 120-581-0665. These days other manufacturers produce the special tool for this purpose. The German tool manufacturer, "Hazet" makes a very nice special tool for this use. It has the 14mm allen at one end which is for service of the standard transmission, rear end and engine oil drain in later 113's. The other end of the tool is a nice 17mm (6 pt.) box end wrench for the engine oil drain in earlier 113's. This Hazet tool has a Mercedes part number on it also #000-589-2407 00. My tool is also dated 03\86. I am sure this is still available and not that expensive. Tom Hanson can probably help you here. Some people can probably use a basic 14mm allen wrench, with a little more difficulty. Good luck.
Joe Alexander
Blacklick, Ohio