My personal timpani include a set of American Drum Company Mark XI instruments manufactured by Walter Light & Sons in 1975, sized 23", 26", 29", and 31". This set of timpani is generally outfitted with natural skins and they are the drums used for the majority of my Orchestra Hall performances with the Detroit Symphony Orchestra. 

 

According to Marshall Light, this wider-hipped bowl shape (relative to the "semi-flat" models which became the standard model of Light bowls) is what came to be known as the "flat-bottom" bowl. Other distinguishing features include a bowl lip that is wider and more round than newer Light instruments, and the absence of lacquer on the bowl, which gives these drums their distinctive vintage appearance.