Most 4 series cases have the small square hole. I've seen a few with the larger oval hole. but not many. All Eaton gear sets have 10 teeth on the pinion gears. They are 18-10, 17-10 and 16-10. Standard units from Chevy are 18-10. Eaton made a heavy duty unit that was also sold by Moroso that had 17-10 gears and the COPO units had 16-10 gears. New Eaton units have 17-10 gears that are made from powdered metal.
Your preload plates look like they have oval holes in them, which would not be in a COPO unit.
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Freddie
1969 Camaro RS/SS396 (427) 4 speed
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