Simply put, stock O.S. head buttons have very little squish band area. The EB button has the taper milled away, similar to Novarossi head buttons. By doing this, the head button now sits further away from the piston. Ed supplies the correct gasket to bring the head button closer and obtain the correct clearance with larger, more effective squish. If you taken plenty of O.S. engines apart, you would be able to know what is different, but then again, most people can't tell if head shims are installed or not.