[John Rogers]

The allen bolts are just regular hardness and can be drilled out with a regular bit or a left-handed bit. Since it looks like a ridge is on the top of the broken section I would recommend removing the bottom half of the mount and using a drill press with a sleeve around the bit as the bit will wobble. If you use a regular bit the broken piece will go all the way through. I am surprised a bolt, just one broke so maybe they are make of an annealed steel on purpose? Good luck.