I'd feel safe in saying mid-80's with the cable guides on the top tube. Likewise the holes drilled in the bottom bracket to allow the derailleur cable to pass through. The foil decal on the seat tube is definitely not any Gitane I've ever seen. If you want to go nice, it'd be worth media blasting and repainting/powder coating and just get another 531 decal to replace the original.
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Syke
“No one in this world, so far as I know — and I have searched the records for years, and employed agents to help me — has ever lost money by underestimating the intelligence of the great masses of the plain people. Nor has anyone ever lost public office thereby.”
H.L. Mencken, (1926)