The roll so prepared shall indicate, as nearly as possible, the group or groups of Indians of California with which the ancestors of each enrollee were affiliated on June 1, 1852. If the affiliation of an enrollee’s ancestors on that date is unknown, it shall be presumed to be the same as that of the ancestors’ relatives whose affiliation is known unless there is sound reason to believe otherwise. Applicants whose ancestry is derived partly from one of the groups named in section
660
(b) of this title and partly from another group of Indians in California shall elect the affiliation to be shown for them on the roll.