One of the things that confuses me about this design is that the anvil could have been more conveniently located on the other side of the vise.
Sooner or later, someone's going to come along and explain what it was/is used for, and we'll all say either, "Oh, yeah!" or "Oh. Really? Wow, that's strange." In the meantime, it's one of the many reasons this is a worthwhile place to hang out.