Two Time Immigrator

Border guards between the Unites States and Canada were not started until May of 1924. Therefore it was simple to cross the border to the other country. Your ancestor's final destination might have been Boston, but they could have easily traveled to Montreal and made their way to Boston from there. In those days the most important factors would have been cost, convenience, and accessibility since the number of ships with passage may have been limited. So start checking the ship manifestos for entries in the other country to see if you can find your ancestor there!