The two plugs(14mm) shown in the photo above are part of the "measuring sensor strip" as documented in MB procedure 20-010 - coolant change.
I may have pointed you at the wrong one. You want to remove and refill thru the 14mm plug that's closest to the front of the vehicle.
As far as a "bleed screw" goes, your 103 may or may not have one. It all depends on what type of t-stat housing cover you have. Some have a bleed bolt with washer atop the t-stat housing cover. Supposedly you can loosen/remove this to bleed air. I bought one last summer and all it did was leak out the washer area - slightly, but nonetheless, a leak. If your t-stat housing cover has no bleed bolt on top, then your best bet would be to follow the instructions in MB service task 20-010. It's worked well for me so far.