I'd put the "decal head tube" models later than mid-70s, but exact date is hard to say, I know that by '78 they were using the decal (not the metal badge) but I have a Super Mistral that supposedly is a mid-70s that still has a metal badge...however, I must say I'm not convinced my bike is as young as mid-70s.
If the 531 decal is original (probably so) the ST (Special Tourist) designation and that style decal came into use around "the early '80s"...maybe '81 is a good working guess.