Now I dont know about BOP rearends but I have always thought that when a rearend was refered to as a 10 bolt or 12 bolt, it was in reference to the cover bolt count, not the ring gear bolts.
Tod is exactly right, always measure the ring gear itself and then you'll know for sure what you have.
Heres a link to a olds only site on their rearends.
http://www.oldsmobility.com/10bolt-tech.htm
Goodluck, Charles