That's a good idea BigFish. I did pull the springs and pistons, and the pistons look perfect, but I didn't think of checking the springs.
>Looks like that had to be a fairly large chunk at one time.
That's what I was thinking. How could something that big get through the filter? I would suspect it would have to have broken off something already in line before the pump, or in the pump itself, but I see no other damage to anything other than damage to the bearings caused by the metal shavings.
It looks like it was caused very quickly, since there's no real wear on the valve plate between the openings in the same area, only some minor galling. Whatever it was, it got chewed up pretty quickly after it did the damage.
For some reason, in the piston bore, around the springs is the only place I could find any metal shavings. I guess that's the only place they got stuck.