"Historian here! I actually took a seminar on those buildings! What you're looking at is the side of a 'magasin entrepôt' (store / storage). They are the iconic grey limestone buildings of the Old Montreal with their huge windows.
"Before the industrialisation, the Old Montreal (which was the centre of the city, and to some extent, the only part of the city (except for the foubourgs)) was mostly made of little shops (ateliers) that dated mostly from the 18th century (1700s) from the mid 19th century (1800s). Most came from the first half of the 19th century.
When the city witnessed industrialisation, these little shops were replaced by the magasins entrepôts and to make construction less expensive, they reused the walls of the older building.
"Those magasins entrepôts had classic tin roof that were replaced in the early 20th century to add one more story. So that's what you see on this picture!
"Fun fact: Quebec city had important union/guilds of artisans, and they were reluctant to industrialisation, so Quebec kept those little shops. So that's why the Old Quebec stayed with the ateliers buildings and the Old Montreal built over almost all its ateliers to make those magasins entrepôts. Most of the Old Montreal buildings were built in the late 19th century and the only few buildings from the late 1600s and 1700s are the religious institutions."