On both my 4.5 and 560 the block drains have copper washers. Don't know the size though. You could always put the car on ramp, drain the coolant into a clean bucket, and take the plug to the store to fit an appropriate sized washer. Then re-use you new coolant from the clean bucket.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.