From where (geographically, class) did Fascist organizations draw their membership? Soucy (is it Soucy?) claims that they had little success luring workers away from the Socialists, but these people still lived in the city, no? Or the banlieu? This should be pretty straightforward, but it might be the first chapter. Go to Soucy to find where one can get membership records. Police reports? Did the ligues keep membership rolls, and have they survived? Has no one answered this question yet?
A more complicated question is whether they were recent arrivals, or had always lived there. Harder to determine.
If you want to claim there was an urban element to this, this is a simple way to do it.