Rob:
No need to take your baby to a luthier. There is nothing wrong with your nut. Just put the nut back in the slot. (Make sure it is oriented correctly, bass v. treble strings). The string tension will hold it in place, no problem. If it bugs you that it is loose without strings, just use a small dab of superglue. As for brass, it provides more sustain and the sound of the open strings is closer to that of the fretted strings than with a bone or synthetic nut. Of course switching it out for a newer Alembic adjustable nut is an excellent choice. Very few guitars and basses sport adjustable nuts - why, I have no idea. It is a great design, IMHO.
Bill, tgo