What Makes This Anvil So Special?

We resurface a 100 year old Kohlswa anvil. Will Stelter and I discovered a neat trick by bouncing a ball bearing to observe how dense the steel of the anvil is. Then we compared our anvils to see which one was bigger and better! This is a big anvil.
What makes an anvil good? Who makes the best anvil? What is a good cheap anvil? How to machine an anvil flat? Do I need an anvil? Is flattening an anvil bad?

Will Stelter’s video