I carry SOB almost always now. It's not exactly on the spine, rather over the kidney at about 5:30.
Go to the above 'Related Links'...search out Fist holsters...see holsters by the number...find SOB #41. It's a single clip holster that is easy to put on and is one-handed removal. You can also draw and reholster with one hand...easy.
It's a palm out draw which is, IMHO, safer then palm in from SOB.
It is very comfortable and I have four other friends who swear by that same carry.
Some don't like to carry over the spine, I don't either. Some feel that you sweep the body on a draw with that position, thus the palm out draw eliminates that problem.
With practice, one can draw almost as fast at from 3:00...and with a button down shirt, I can have my hand on my weapon for the draw with no one seeing what I am preparing to do.
Stay armed...carry in a way that works for YOU...stay safe!