This date was added in 2010 by the Gender Equity in Indian Registration Act, which is an amendment of section six of the Indian Act.
That bill was passed in response to the McIvor decision, which found that the provisions at the time of the Indian Act were contrary to section 15 insofar as it treated differently the male and female lines with respect in particular to individuals affected by the double mother rule. That rule was introduced in 1951, which is why the 1951 date was introduced in the remedy.