The Puranic lists of twelve Adityas are not identical, and Amsha sits precisely on the fault line. In the list preserved in the Vishnu Purana, where one Aditya presides over each month of the year, the ninth is given in Wilson's translation as Ansu, and the same list has Parjanya, the rain god, in a place that other sources give to somebody else. The Bhagavata Purana's list of twelve has no Amshu and no Parjanya at all, and puts Savitr and Vidhatri in their places instead. Ten of the twelve names are common to both; four are not.
The identity question follows from that. Sanskrit amsa, a share, and amsu, a filament, a ray of light or the stalk of the soma plant, are different words. A Puranic list that has become explicitly solar, with one Aditya to a month, has an obvious reason to prefer a name meaning ray. Whether the compilers of that list were spelling the old Rigvedic name loosely, or deliberately substituting a solar name for it, or recording a genuinely separate figure, is a question the surviving texts do not answer.
The atlas records the two spellings on one entry and marks the identification as open rather than choosing. The entry carries an open question naming what would settle it, which is a comparison of the readings across the Puranic manuscript traditions rather than any new discovery.