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Looks wiped like from a momentary loss of oil pressure or from low pressure at start up. I would start by cleaning the caps, the block registers, bearing inserts and crankshaft. Then I would either mike them or at least use plastigage to see what the clearances look like.
Were the main saddles checked for round, or honed?
Carefully cut open the filter to see what it has picked up- this will likely show you what caused the grooving seen on some of the bearings.
Get a look at the back of the bearings to see if there looks to be anything caught between the bearing and cap or block.