The Peshito (P'shitta) or " simple " revision of the Old Testament is a translation from the Hebrew, though certain books appear to have been influenced by the Septuagint.
But both these were Monophysite and of limited use, and the Nestorians still went on using the Peshito.
It seems certain that the Old Syriac version also contained the Acts and Pauline epistles, as Aphraates and Ephraem agree in quoting a text which differs from the Peshito, but no MSS.
The Old Testament Peshito is a much older and quite separate version.
It seems probable that the Old Syriac version did not contain the Catholic epistles, and as these are found in the Peshito they were presumably added by Rabbula.