After clicking the button on the page that lists the Forms 1099-R that you have entered, when you indicate to TurboTax that you made nondeductible traditional IRA contributions, TurboTax will ask you to provide your basis in such contributions. You would find this amount on the next-to-last line (usually line 14) of Part I of the most recent previous Form 8606 that you filed.
(No Form 8606 could have been filed in the '70s. Form 8606 came into existence in 1987 when the tax code changed to permit nondeductible traditional IRA contributions.)
