Thanks to everyone's help here, I bought an Enco 4" milling vise. I unbolted it from its swivel base, since it's one more thing to not be true and it steals precious inches of height (I have a small mill/drill without much vertical clearance).
I want to mount the vise directly onto my mill/drill table. But after removing it from the base, there are two rectangular keys protruding from its bottom, about 1/2" square by 1" long. The keys are held in with bolts, but they also appear to be held in by punching the join between the key and the casting. When I remove the bolts, the keys still won't come out, even if I whack them laterally with a mallet (using a piece of wood to avoid denting anything). I didn't try whacking with all my might.
What's the right way to remove these, without damaging anything?