Earlier Cinelli stems were 26.4 mm. But later some (not all) Cinelli stems were also being made in 26.0 - which had become the more universally accepted road bar diameter by the 1990s.

Traditional 3TTT quill stems were always made in 26.0 mm.

As I recall, the Mutant was an early front-loading road stem. They were made as BOTH quill stems for Threaded steering tubes and soon were also offered as Threadless stems (for 1" threadless steering tubes).

I thought they were all intended for 26.0 bars. BUT... I seem to recall that 3TTT had also begun to make handlebars with a 25.8 mm diameter a few years back - probably an odd attempt to bridge the gap between Road and MTB stem/bar diameters.

So, you may have a stem which was originally made for 3TTT 25.8 mm bars. Perhaps you could file the stem's clamp area a bit?