I have heard California built 442's in 66 came with 12 bolt rear axles ( 12 bolts as in cover bolts, not necessarily ring bolts)

 

Oldsmobile started using the Type O axle under the Cutlass line in May of 1966. The Type O has a 12 bolt cover but only ten bolts holding the 8.5" ring gear. It is not a "12 bolt" and no Chevy 12 bolt parts will fit. This was not a function of being built in California - Lansing cars were first.

All Cutlii built in CANADA for 1968-69 DID get the Chevy 12 bolt in place of the Type O, however.

 

Thats interesting..my car is a late May 66 build and has a 12 bolt COVER. Is there any casting, or stamped dates on the rear ends?

 

Which is consistent with when Olds started using the Type O axle. All assembly plants did this, not just Fremont. You won't find any info in the Chassis Service Manual since that manual was printed in Oct 1965. There is a dealer service bulletin from May 1966 that talks about this.

 

The julian date on the rear is 170. My car was built 05D. Doesnt work...UGH

 

Why doesn't it work? That's the last week of May which would be in the 180 range.

 

Thats end of June

 

My bad,sorry.