I saw the Snap On truck at a shop, stopped and bought a 14MM, 1/2 inch drive Allen from him. He had it on the truck. That was probably 12 or 13 years ago.
It is the right size to fit the transmission (manual) drain plug and differential drain and fill plugs. However, it won't fit up between the transmission fill plug and the tunnel. You will need the special drain plug wrench, or you can mess up the outer portion of the plug with Channel locks and get the job done.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in