It's two rows of ribs, same as Cher, although hers where supposedly for a better figure*
first hit on a Google search, includes
" Marilyn Manson supposedly had his bottom two ribs removed, so that he can perform oral-sex on himself. He bragged about this in interviews, although some say he made that up and he never actually did this. Just the fact that he would state such a filthy thing, shows his reprobate mind."
I
really hope that the linked site is a spoof
*some crap on Channel 5 about this included an interview with a plastic surgeon saying that this is damned near impossible without massive scarring.
Edit - Rebecca got there first.