Nice looking lump of iron. From what I can see in the pictures, the holes are located in areas where there are not stiffening ribs on the back of the casting (that's a guess). Any hole drilled elsewhere would run into the ribs and would be around 3" deep to get all the way through. Possibly not a great arrangement. You could certainly make the holes bigger if you wanted. Although 1/2 " is probably big enough, a lot of the fixturing tooling is 5/8" IIR. You could also tap some of the holes to accept bolts from the top. If the holes aren't tapped or equipped with some other arrangement, you'll have to reach under the table to put a nut onto the bolts from above. If it was mine, I'd tap the holes at the corners of the patterns and see how that works out. Not a big deal to do more later. You could also consider drilling and tapping into the ribs that I speculate are on the backside. AFA what was it originally for or who made it, not a clue.
