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Let $K$ be a number field. Let $S$ be a finite set of places of $K$ containing all the archimedean ones. Let $R_S$ be the ring of $S$-integers of $K$. In the present paper we consider endomorphisms of ${\mathbb{P}}_1$ of degree $2$, defined over $K$, with good reduction outside $S$. We prove that there exist only finitely many such endomorphisms, up to conjugation by ${\mathrm{PGL}}_2\left(R_S\right)$, admitting a periodic point in ${\mathbb{P}}_1\left(K\right)$ of order $\>3$. Also, all but finitely many classes with a periodic point in ${\mathbb{P}}_1\left(K\right)$ of order $3$ are parametrized by an irreducible curve.