Difference between Amperex 6922 pinched waist tube

I wonder if there are differences in the Amperex 6922 pinched waist gold pin tubes that I have heard about. I see some were made in Holland and some were made in the US. Is one supposed to be superior to the other. I have read that they are pretty much the holy grail of Amperex tubes. I just picked up some Siemens CCa tubes and really am impressed but figure I may as well see what else is in their league. Thanks for any advice as I could always use it.