Looking at the pic carefully, this looks like a cronic overheating problem that is at the fail point.
The discoloration of the "b" phase fuseholder, and the fuse are usually caused by excessive heat. As the fuse clips heat-up, they get metal fatigue, and loose the spring tension, further aggravating the heating situation.
The "metal clips" are spring steel, that is factory installed on "A" & "C" phases; the piece on "b" looks like a "band-aid" fix that didn't work well.
THe fuse mfg's (Bussman) used to make a fuseholder clip retrofit for clips that had failed spring tension. I don't know if they are still available; they were OK IF you caught the tension failure early.
I would like to see the "B" fuse out of the fuseholder, to see if that's a "split" in the copper or some fancy attempt at machine work.