and the "hardy" hole is usually square, not rectangular as is the item pictured. Hardys could be fullers, hot chisel, cold chisel, formers, etc.
as before mentioned, a "hardy" was a tool that fit the "hardy" hole. Anvils also have another hole, it is usually round and it is called a pritchel hole.
""In the heel is a square hole called the hardy hole, which (would you believe?) holds the hardy. Hardies are a group of tools used to cut, swage, fuller, flatten, or shape bar stock. The blacksmith puts the hardy tool on top of the hardy, heats the stock in the forge, places the hot stock on top of the hardy, and strikes it with the hammer to forge it into the desired shape.
Also in the heel are one or two smaller round holes called pritchel holes, used for punching holes through bar stock or horseshoes with a (pritchel,) or punch, without damaging either the face of the anvil or the pritchel."" from
http://www.horseshoes.com/advice/bstall1/anvils.htm