I think prior to that canada descent rules were pretty lenient and could skip a generation.
On the contrary; when Canadian citizenship was first officially established in 1947, those that were born abroad in the first generation (before and after 1947) had to apply for retention of their Canadian citizenship and renounce any other citizenships by their 22nd birthday. This rule was later relaxed to the 24th birthday and abolished altogether in 1977, but those who were born before 1953 (such as your mother) and failed to apply for retention still lost their citizenship. 1977 also brought about the allowance of multiple citizenships, but it did not restore citizenship that was lost because of the previous legislations. Anyone who lost their citizenship for any reason except renunciation and fraud had it restored automatically in 2009 and 2015, but the generation limit was also implemented at the same.
Until 2009, descent was based on your parent's status only, not your grandparent. At the present day, the only way to skip a generation is if your grandparent was a government worker or serving in the Canadian armed forces at the time of your parent's birth. Unfortunately, if your grandfather was not in such an occupation when your mother was born in the US, then you can not take advantage of this exception.