That's a good question. The Gibson mitt design was patented in 1913 and the snap button heel in 1912. There is an illustration in a 1913 catalog of the original Gibson mitt like yours with no snap heel. Unfortunately I haven't found Rawlings catalog images from 1914 to 1923 on the website:
www.baseballglovecollector.com. Joe Phillips new Rawlings Catalog Source Book lists the original Gibson mitt along side the snap heel Professional model in a 1922 catalog. I would speculate that the Gibson mitt with snap button heel was introduced between 1914 and 1921. I hope to find a catalog illustration some day.