Could there be a M-type asteroid beyond Jupiter? Is there any reason to think an M-type asteroid could not have migrated beyond Jupiter and entered an orbit similar to the Centaurs?
According to the Grand Tack Hypothesis, in the early solar system, Jupiter and Saturn migrated inwards and then outwards, scattering asteroids as they did so.

In this hypothesis it seems inevitable that some m-type asteroids will have ended up in orbits with large semi-major axes.