Manuals are available here:
http://www.mercedesmanuals.com/Mercedes_Benz_W_126_Manual_p/w126.htm
Don't feel bad, a transmission shop did the same thing, the mechanic, I've never seen a transmission that only holds a couple of quarts. This is after I told them they need to drain the torque convertor, they said "we'll get it, we'll get it".