Thanks a lot for all the helpful replies. I tried to measure these things again. The handlebar is definitely 26.0mm. Yet, I do not have a precise way to tell the clamp diameter of the 3t mutant. Seems to be around 24.5-25mm. Anyway, as stronglight pointed out, the Mutant is an early front loading stem (and pretty sweet looking indeed). As I take of the front plate, put in my handlebar, and then reattach the front piece, there remains a 2mm gap. Might probably work, but since this is my main road bike, durability is important for me and I wouldn't like to be surpriced in the hills.

So, do I get it right that as a rule most of the 3t stems were 26.0 and the Mutant could be some kind of an exception. I would then probably try another earlier 3t stem.