Do you have a picture.   1964 and 1965 beaumonts and chevelles made in canada never had mirrors put of from the factory.  they were all dealer installed accessories.  Unless it was a remote control mirror.

Alot of customers at that time were still mounting mirrors on fenders, doors, some wanted the GM canada "chevrolet" mirror, some had the pontiac style ones,   it was sort of anything goes when it came to a dealer installing a mirror on a new beaumont.  I have seen just about every configuration.   And i have owned several cars that have never ever had a mirror on them, due to beeing original unrestored cars.

I would say, your choices are wide open for the mirror.   If i was you the most common was the 1964-64 chevelle mirror but buy the one that has NO bowtie,  that would be the exact replacement for a canadian mirror.

But if you prefere, you can also put on a 64-65 GTO style one,   being a beaumont used alot of pontiac styling cues