Here are the links to three of the greatest auctions dedicated to strictly inverted center stamps! I have posted the links below. Each was an exceptional collection.
The Peter Balner Inverted Center Stamps of the World
The Rober Cunliffe Inverted Center Stamps of the World
The Mahendra Sagar Inverted Center Stamps of the World
http://www.cherrystoneauctions.com/_auc ... ion=201110