L6 Fused vertebra

The rest of us have L6 vertebrae that fused to become a part of the pelvis eons ago! If you look at the sacrum, you see holes that nerves exit from - these are the vestiges of gaps that in evolutionary history separated distinct vertebral bodies.

There's no telling whether your L5-S1 disc has wear and tear because of your particular genetic variation or because of the usual forces that also act on those of us who don't share your particular history. I, for one, had a huge amount of degradation at an early age in my L5-S1 disc - all with no special history involving an L6 vertebra. But it is possible that you are more disposed to this kind of degradation, just as it is possible that I am geneticlaly predisposed to have a problem in this location for another reason (many of the members of my family have had sciatic pain, though as they have begun signing up for our Gokhale Method Foundaions course, they are also happily experiencing relief).

I don't think your pathway will be significantly different from what you would do if you never had a discrete L6 lumbar vertebra. Your emphasis should still be on lenghtening and stregnthening, with perhaps more care put into how and how much you antevert your pelvis.  recommend working this step with a qualified teacher.