Probably early 1980's given the Shimano 600 components. If you look 531 frames from Europe's mass producers of that era, like Puch/AD, Peugeout PX10, Raleigh Comp/Int, and Motobecane GR, clean examples sell between $250-$750, and more in rare cases. So maybe the Allegro is worth as Lenton58 has said, $350-$400?