In his patent from 1900, Willie Dunn came up with the concept of a negative loft putter. This borrowed a concept from billiards whereby the player intentionally hits the top half of the ball which imparts the kind of roll known as a "follow."
According to The Clubmaker's Art, only 5 gunmetal Willie Dunn "Rotary" putters are known to exist. Negative loft and a convex face with a flanged sole and hollow back. A very unique putter.