this one is 2" by 2" across the anvil top plate with an inch and a quarter hardie stem and the stem is offset away from the notches (just so it fits, I think). the others are 1.5", 2" and 3" across the top, I think that is deliberate.
On this one, one notch fits 7/8" round and the other 9/16".
The octagonal shape of the stem had me thinking that it might fit diagonally and go over a pritchel hole, but it looks like the bloke that made it has accidentally made the stem undersized and reworked the metal out of the corners to salvage it. The others are all 7/8" stems to go in a hardie stand, not the hardie hole in an anvil.