Okay, here's an update. In both photos, it is bolt #2 that is the block drain as Shooter said. It takes a 6mm allen socket. Each bolt also has a sealing ring that needs to be replaced. The sealing ring is part #007603-014104. It looks to be about a 16 x 20 x 2 aluminum ring, if I'm measuring it right.

I was able to break the one on the driver's side free and a lot of coolant came out. The one on the passenger side won't come out. It looks like someone had a go at it at some point and rounded off the edges so that my 6mm allen socket just turns in the head of the bolt. If anyone has a recommendation on how to get it out, I'd like to hear it. My thought now is to just leave it be and flush the system a few times. I was able to get about 2.5 gallons drained. The system holds about 3.5 gallons, so a couple of flushes ought to get most of the old green stuff out.

__________________
Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles