The first year for that downtube decal was 1971. Then, in 1972, they changed from the fancy Nervex Professional lugs to the simpler Nervex Dubois lugs. In 1973, they moved back to the fancy lugs, but this time the 531 decal was moved to the down tube from the seat tube. So, having the fancy lugs with that downtube decal, and the 531 decal on the seat tube, this is a 1971 PX-10.
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My collection: 1947 Ciclo Piave, 1955 Liberia, 1969 Colnago Super, 1972 Legnano Olimpiade Record Specialissima, 1980 Mercian Vincitore, 1983 Gitane Interclub, 1985 Peugeot PGN10, 1985 Eddy Merckx Corsa, 1985 Hood Cycles Lo-pro, 1986 Bianchi Vittoria, 1987 De Rosa Professional, 1989 Vitus 979, 1989 Bianchi Super Leggera, 1990 Bianchi Axis, 1990 Specialized Sirrus, 2001 Colnago Dream B-Stay, 2007 Trek 1000