Is it possible that an early September build date 67SS left the factory with a 3855961 block (August cast date)? Any documented cases?

67 blocks (3902406) were already being cast in August, there is no reason to expect a 961 block to be cast then. Are you sure the casting date is August 66, not August 65? The 5s and 6s are hard to tell apart.

I might expect to see an early built 67 with a left over July 961 block, but a August 961 seems unlikely.

Tracy Performance has a 3902406 block dated H 1 6 (August 1, 1966) in it's list of blocks for sale.
http://www.tracyvette.com/blocksbb.html

Thanks for the reply. I think you are correct.