Far more common than a 531 frame is the entry level Jeunet, which is gussied up a bit. It has the half chrome rear stays you often see on higher end Frenchies and the aforementioned Simplex drops. However, the frame is anything but 531; there's a nice seam down the inside of the head tube, and the post, IIRC, is 25.4. In addtion, every single one of this model I've ever seen has fully chromed forks, whcih look like replacements but aren't (catalog pics from the time show the forks as half chromed, but I've had four or five of them in my hands and they've all been full chromed, as well as all the ones I've seen on the bay. This lower end model has cottered cranks, as well.

Way back, when vintage bikes were cheaper, I stupidly thought twice about a 531 Jeunet on ebuy that was only 150 plus shipping. Stupidly, I didn't BIN it. One of my few bike buy regrets ever.