All of the drain plugs on the 126 tranny pans I've seen feature a small round plug made out of metal. In the center of the plug, you should be able to fit a 5mm allen head wrench.
As was previously suggested, you may wish to remove it and replace the flat washer - available at local dealer. You'll drain approx. 3.5 qts of dexron and that's easily refilled thru the tube where you check the tranny fluid level under the hood. The actual amt. drained may depend on the previous content.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.