I am looking at a mirage frame locally (I suspect that it is an early 70s) and I am tad confused. From what I have seen in the internets, these were 10 speed rigs with a simplex RD. This frame looks like a track frame (no RD attachment built in) and the spacing is closer to 126 than 120 (with my little pocket measure), which is tres strange for a 5 gear cassette bike. It does not look like a hack job because it is ultra smooth and has the original paint job. I've seen several Mirage frames from that era on line and all look like (well) track frames. Did those bikes use a RD bracket of sorts?
Mirage frames from the early and mid-1970s use stamped road dropouts. These dropouts do not have an integral derailleur hanger, however the axle slot opening faces frorward unlike track dropouts where the axle slot opening faces backwards. They require derailleurs with a built-in claw adapter. Frankly, I don't ever recall seeing a Mirage with a Simplex derailleurs. Typically, they were Huret Allvit equipped until circa 1974 when they switched to SunTour VGT for the remainder of the 1970s.
As for the spacing, it can vary, depending on the hubs. Many French hubs were actually 122mm or 124mm over the locknuts.