If that's the back side of the #3 main bearing, you're seeing wear on the thrust face, probably from a poorly-adjusted throwout bearing keeping a constant forward pressure on the clutch. Check the end play of the assembly when you reassemble it with new main bearings to check for thrust face wear on the crankshaft. A good machine shop that does crankshaft grinding can probably look up a specification for allowable front-to-back distance between the thrust flanges of the crankshaft, also.
Jerry
