You can't believe how much of a headache you get when you're tired to begin with and you try and distinguish the 'real' and hypothetical existence of six gospel narratives all the while trying to follow the development of particular sections of the text. Oh God! Nevertheless from looking at the Diatessaron I can see why the Marcionite emphasis of the seventy two REPLACED the commissioning of twelve men that formed a body called 'the twelve' in other gospel traditions.
The Diatessaron places the story of the sinful woman who is 'about twelve' in the exact same place as the discussion of the twelve apostles.
I suspect the juxtaposition was deliberate.
Keeping in mind all the things Irenaeus says about the Marcosians let's note that Christ is the 'six,' the woman (who I suspect is Marcus Agrippa's sister Berenice aka 'Mary Magdalene) is the 'twelve' and the 'seventy two' are the Marcionite equivalent of the twelve apostles who as I noted don't appear in the Marcionite gospel (how could the Marcionites have understood Peter to be a pseudo-apostle if he was the head of a body of apostles commissioned by Jesus)
Oh and yes, for my American evangelical readers I should make it explicit that 6 x 12 = 72
And while we are at it 12 x 30 [the number for Jesus and Judas in the gospel i.e. y (10) h (5) u (6) d (4) h (5)] = 360 the number of days in the sectarian calendar for Dositheans, Montanists, Egpytians etc.
Stephan Huller has had a life long interest in religion and spirituality 
dating back to conversations with his grandfather, Gaston Frank. "He said we represent one of the last descendants of the Frankist Jewish faith in the world," he muses. "I grew up thinking that our family was something like the Last of the Mohicans."