Don't know about a 1971, but 1969 RE is for a 3.23 open diff.
In 1968 it was used to describe a type C rear axle (I think the C is for Chevy or maybe "c" clip???) with 3.31 gears.

I don't remember seeing a 12 bolt with 3.23 gears, so guessing it is a 68 housing.

Can you read the casting date code on the housing? Also, the casting number would be a clue.

Not unusual to find a chevy 12 bolt in a late 60's Olds.