David Zimkin lists Arthur J. Zimkin as next of kin
Ancestry has a host of records for David Zimkin, husband of our great aunt Sadie. We see census records for 1900 and 1910 as well as the 1920 and 1930 we already knew about; draft registration cards for both World War I and II; and his Social Security death index. He died in New York at age 80 in 1966. (Note that he was born in Riga and that there is lots of information available about Zimkins in Latvia, but I don't have time for that now.)
Here is his WWII draft registration. Notice that he lists his only son Arthur J. Zimkin as the "person who will always know your address." Why is this significant? Because it is the first confirmation of Arthur's middle initial, which further links him to that Montreal engagement.