Been awhile since I removed an OM603 head, but I recall the same problem, and "found" yet another headbolt that I had missed previously.

I think there was oil on the top of it and the bolt may have been in a slightly recessed location, making it difficult to spot.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles