Doubt it is oil contaminant related as spalling/fretting seems to be limited to one bearing and in one area. Unless you wet sump heavy and wet sump often, it is really difficult for any significant large contaminants to get back there as these bearings remain happy with only a misting of oil. Furthermore, roller element bearings are tolerant of contamination. Just consider that this spalling managed to pass from within the bearing without doing more damage.
I doubt it is brinelling (maybe false brinelling). It looks like the damage is localized to maybe 10 degrees BTDC.
I had experienced a similar failure (though on the drive side and I do not recall the orientation) back in the day (1970s) while running a Drouin with 10 psi boost. I attributed the failure to my carelessness in overheating the inner race of the new roller mains or something along those lines when rebuilding the motor for the blower.
Upon your dismantle I would take note of the class of fit of the bearing as well as how tight it was on the main shaft as well as the outer fit in the cases - maybe too tight. Also take a look at the main shaft and inside of the race to see if there were any upsets of metal causing a tight spot.
You may never be able to determine exactly why it failed but most importantly, whatever you do, don't do it again!