Oswald - Just out of curiosity, when you checked the other main bearings, did you happen to look at the upper half bearing shells to see if they had the notch and tab design suggested by Kevin above?

Just grasping at straws here to see if maybe in the engines past someone used the mix and match parts method to replace an original bad bearing, or maybe as Joe H suggested, the rear main cap got swapped with something that wasn't checked for proper fit???

Jerry (HRL) can probably answer better on what may have caused the odd wear on the rear main lower shell, but if it was just me noodling the situation, I'd wonder if the rear main cap maybe wasn't properly shimmed for the crank/bearing/cap clearance as one possibility. The concentrated wear towards the rear of the bearing to me seems to indicate the rear main bearing surface either wasn't the same front to back, or the rear main cap wasn't checked/matched to the crank or maybe was improperly shimmed for proper bearing clearance.

Just trying toss ideas out in case it may be of some help.

~ Dan
1951 Chevy 3 window 3100
Follow this story in the DITY Gallery
"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)