From the Boulevard, this structure looks like a respectable Haussmannian stone building. It is a little featureless and dull, but also clean, neat and comfortably bourgeois. Take the Rue Bayen though, and a surprise awaits. The rear of the building is completely different! From this perspective, the building is pure red-brick proletarian (although with lines and forms that I personally find more attractive).
Return to the Boulevard and take a step backwards and the full picture becomes clear. The bourgeois respectability seems to be in fact little more than a clip on stone facade.
In fact, such masquerades are comparitively common in Paris, but often much more difficult to spot. As Bernard Marrey writes in 'La Brique à Paris', "si les façades sur rue cherchent...à exprimer une 'position sociale' supérieure, la façade sur cour ne cherche rien, sinon, parfois le simple plaisir d'être entre soi" (if the street-facing facades try to express a superior social position, the courtyard facade has no such desire, if not just the simple pleasure of being with its own kind).
Marrey also points out that, as is the case here on the Boulevard Pereire, that residents had separate entrances, and that those who had an apartment with the stone facade would also surely have other different touches, such as an elegant staircase. They may live in the same physical building with those facing the rear, but they couldn't risk meeting on the stairs!