I agree what AS109 said, the purpose is to give you multiple options for clamping different shaped objects including round tube without damaging the material.
Very nice find......Last time i seen one of those was back in my teens at an Industrial Museum somewhere in either Michigan, Ohio or Illinois but dont remember where Exactly. I remember because I asked the tour guide about it specifically and what I stated above was his reply.
What I find very interesting is how very thin the "Anvil" face is! I wonder if it is that thin because of too many "resurfacing" treatments with a grinding wheel?
After taking a closer look the hole almost looks like it was purposely put there because of the shape, what do you think about it?