68/69 Chevelle 12-bolt housing casting numbers

[Mr70]

I can't answer your question,but on the other end of this they used the #038 bum in a few early assembled 1970 Chevelles too.Not like the more commonly found #3969278 NF.
My 70 L-78 has it's original 4:10 CKK rearend dated late June of 69.A scarce few other 1970 Chevelle L-78 4 series optioned owners have them within this time period too.
So far I've only encountered it when the 402cid/375HP was used with the 4:10 ratio,not 3:55 or even the 3:31.It looks like Chevrolet wanted to use up the previous years inventory as they were phasing out this particular rare engine/rear ratio combination.
I am curious of the timeline others offer to your question.