Well here goes. As originally introduced the little H & R hammer gun was offered in 28-gauge and .44 caliber. A 1909 vintage magazine ad --



By the 1912 SD&G catalogue they had added the 20-gauge --



By H & R Catalogue No. 13, circa 1920, the line is filled out with the addition of the 14mm/32-gauge --



By H & R Catalogue No. 15, circa 1924, the little hammer double is gone.