Of course very little of what the Marcionites actually believed or practiced has come down to us so it is difficult to confirm that they indeed possessed a hidden 'mystic gospel' in addition to a simply historical narrative. Nevertheless words attributed to Marcion about his community's sacred text makes it difficult to ascribe them to our rather poorly written canonical gospel-type - "O wonder beyond wonders, rapture, power, and amazement is it, that one can say nothing at all about the gospel, nor even conceive of it, nor compare it with anything" (von Harnack, Marcion, so Anm. 2, 256. 1 18. 94 f.; Schäfers, Eine altsyrische antimarkionitische Erklärung von Parabeln des Herrn usw., 1917, S. 3 f.)
This reference comes from an anonymous Syrian commentary on the gospel which identifies the 'Proevangelium' of Marcion as beginning with these cited words. Harnack wrongly assumed that this was the same as the so-called 'Antitheses.' Sebastian Moll agrees noting that Harnack:
was convinced that this Pro-Evangelium referred to Marcion's Antitheses. However, the enthusiastic opening statement just mentioned does not seem to fit what we have discovered in this chapter concerning the rather monotonous character of the Antitheses. The only other work of Marcion known to us is his letter, but that is even less likely to be identified with the Pro-Evangelium. Therefore, this Pro-Evangelium is either a third work of Marcion we do not know anything about, or, and this is the option I would suggest, the name refers to nothing else but Marcion's Gospel itself. Everything fits so well. We are dealing with a commentary on the Gospel, so it would make perfect sense for the author to refer to Marcion's Gospel - as the competing one - at the beginning of his work, rather than to any other work written by the arch-heretic. Moreover, right before the author mentions Marcion's Pro-Evangelium, he declared that all those writings are untrustworthy which are not based on the Law and the Prophets. This critique again applies perfectly to Marcion's Gospel, since it was free of any positive reference to these texts. Finally, the name Pro-Evangelium (in the sense of 'prior to the Gospel') would be most appropriate for Marcion's Gospel, as he indeed believed his version to be prior to the one used by the Church. [the Arch-Heretic Marcion p. 119 - 120]Moll adds that that the opening words to the Gospel are never mentioned by Tertullian or others may simply be due to the fact that they were added later on by Marcion's followers.
It should be noted that what has caused everyone problems including von Harnack is the underlying assumption that the Marcionites had only one gospel. If we can allow ourselves to conceive of the possibility that the Church Fathers were ill-informed, we can embrace the anonymous Syriac report as proof once and for all that the Marcionite gospel and 'mystic Mark' were one and the same.