As Souter notes:
There was certainly some kinship between the text used by Ephraim and that used by the heretic Marcion. This need not surprise us. Tatian and Marcion were in Rome about the same time. Just as Tatian used a Western text of the Gospels as the basis of his Diatessaron, so must Marcion also have used a Western text as the basis of his recension of St. Paul s Epistles. Doubtless Tatian brought the Epistles of Paul to Assyria, and may have translated an early Western text of them for the benefit of his compatriots. It may have been Marcion s edition which was translated in the first instance, and afterwards amplified. Of this text we would gladly know more, but only Armenian experts can give us this, and, so far as I know, they have not yet done so with the desirable fulness. [Souter Text and Canon of the New Testament p. 59 - 60]
Ulrich Schmid clearly showed evidence of Marcion's text being related to a 'pre-Western' text in his Apostolikon. More recently Dieter Roth adds "[b]oth Marcion’s Apostolikon and Euangelion reveal affinities to the so-called ‘Western’ textual tradition, though the text is definitely not the ‘D-text’ and likely represents a precursor to the ‘Western’ text."