The term common stock as used in sections 51a, 51b, 51c, and 51d1 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 1781 of this title, shall mean only the amount of common stock outstanding.