A pictorial cancellation dated 29 November 1945 adviced senders to add the arrondissement number on adresses to Paris, the twenty parts of the city, so that mail can be sorted quickly. French mailboxes usually are divided in two cases: local mail and far away (other départements or foreign destinations).
The datestamp was used at the P.L.M. railway station's post office, understand the gare de Lyon, the starting station of the former network "from Paris to Lyon and to the Mediterranean Sea". A station I love because it has been the first step to my recurring return to the blue sky of my chilhood.
If you walk in this station, remember to see above the sale counters to watch the fresco depicting monuments and landscapes of cities where the PLM tracks were conducting you.