That looks like either a mixed tubeset or plain gauge 531 sticker -- can you quote the writing on it?
The seat post diameter will tell you whether it is double-butted 531 (26.4mm for a French bike with 28.0mm OD seat tube) or plain gauge (about 25.8mm).
I wonder if Mercier made a model between the 300 and the 100, i.e., perhaps something comparable to the Peugeot
PR/PKN-10.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069