Cork has been used for many applications in the game of golf, but offered here is the unusual application of cork to a clubface. Lewis Anderson received a British patent for this idea in 1914, and Spalding released a club apparently using Anderson's patent the following year. According to The Clubmaker's Art by Jeff Ellis, "[d]espite being advertised from approximately 1915 through 1920, Spalding's 'Cork Center' putter was perceived as a gimmick and was not popular."
Well, at least that explains the rarity of this putter 100 years later. The putter head is brass or gun metal, and the face is entirely cork.