I think that the nut is original to that bass.
I once had a short scale bass that I sent back to Alembic for a neck repair. I thought that since they had to remove the fingerboard, that they could replace it with a fretless fingerboard. So they did. When I got the bass back from Alembic, they also installed a bone nut. When I asked about this, I was told that it gives a more consistent tone between a fretted and open string being a fretless instrument. Makes sense I guess.
They sure did an incredible job of converting that bass to a fretless.
