I always tell that the square hole is late 17c or later so it would be nice to say that the anvil I use at markets and rennfairs could be period. Not that i belive it is, but anyway... :)
http://www.anvilmuse...llection-1.html
This is the oldest anvil i found in collections with a square hole in it.
Did not belive that it should be so hard to find "The History of Anvils"