Matching postal codes to delivery boxes has been far easier so far than I had expected (south of Fauna Street, anyway), except that it turns out from this I have utterly messed up when I originally noted the names of Émilois Street (formerly Giroux Street) and Françoise-Giffard Street (Formerly Laurentian Street). I have now fixed that.
My original memory--which puts Giroux (and not Laurentian) intersecting with Lapierre--and not the maps, were accurate, Canada Post's database currently puts these addresses firmly on Émilois Street.
On the way, I noticed no less than three mail boxes I had overlooked: one on Beading Street (amusing fact: despite this, and that there are car entrances on the street, no addresses are actually located on Beading Street!), one on Lafond Street, and a second one on the Fréchette Street frontage of Beauséjour Park (that last one did not exist in my memories!).
I also came across the SAME employee at the Beading street mailbox XD. Only one last anomalous thing is that CP gives a G3E2H4 code for a segment of White Poplars Street, which makes no sense in any case: there is no room for an extra box (I have now walked the whole street! Twice!) and there is NO reason for it to be a G3E 2** postal code there!