Good answers guys, but I don't think they're the ones I'm looking for. I know of the V grooves that are in some drillpress type vices that are on the face of the jaws. Guess I should've referred to this as a square slot, and in pictures on large heavy-duty mill vices, it will run horizontally across the top of the jaw. A Kurt D688 has this type of jaw, per the MSC flier.