I popped the hood on my '91 300-SEL. I see the 2 valves you outlined in your photo. Mine have MB parts numbers and some other identifier on the bottom side. It's dark and I was using a weak flashlight. The numbers appear to have faded with age.

I will take another look when I have better light in the A.M. and let you know what I do or don't see.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"