But apparently only in loaded ammo. Hopefully (maybe with a little pressure from consumers) they will offer them again as a component. Great hunting bullet!
I think you are correct. Nosler discontinued a "solid base" bullet years ago and I suspect the NBT took their place. I cross sectioned a number of bullets and I believe that the special tapered jacket used in the BT's is the same they used in the SB.
I picked up a large batch of 6mm NSB's as seconds and they were great hunting bullets. The only difference seems to be the solid base is lead tip, not plastic.