The term “common stock” as used in sections 51a, 51b, 51c, and 51d ¹ of this title means stock of national banking associations other than preferred stock issued under the provisions of said sections. The term “capital” as used in provisions of law relating to the capital of national banking associations shall mean the amount of unimpaired common stock plus the amount of preferred stock outstanding and unimpaired; and the term “capital stock”, as used in sections 101, 177, and 178 ¹ of this title, shall mean only the amount of common stock outstanding.