For example, take the descent of Jesus from above already adult, in the incipit of proto-Luke. It could be an invention by Marcion, or an invention by a different person, with equal probability.
But totally beyond the identity of the his author, it is a fact that "Mark" (author) interpreted the descent from above as being a marcionite thing, and precisely in virtue of that interpretation (beyond if right or wrong), he removed it by introducing John the Baptist in the incipit.
Hence, even if Marcion didn't write proto-Luke, to all the practical effects it's like he wrote it.
The great irony in all this is that "Mark", "Matthew" and "Luke" were the earliest proponents of a "Marcionite priority", remote precursors of prof Vinzent, since their respective versions of the same story reveal de facto that their interpretation of their common source (=proto-Luke) was marcionite in essentia.
The historical "Mark" is Markus Vinzent.
