In the fifteenth year of the reign of Tiberius Caesar Jesus Christ, Son of God, descended from heaven and appeared in Capernaum, village of Galilee...
Implicit in the mention of the precise date of a mythological event is the claim that someone witnessed the descent of Jesus Son of Father ("Bar-Abbas") on the earth. For Marcion, that particular witness could only be: Paul.
The Judaizers interpolated John the Baptist in that same incipit:
In the fifteenth year of the reign of Tiberius Caesar—when Pontius Pilate was governor of Judea, Herod tetrarch of Galilee, his brother Philip tetrarch of Iturea and Traconitis, and Lysanias tetrarch of Abilene— during the high-priesthood of Annas and Caiaphas, the word of God came to John son of Zechariah in the wilderness.
(Luke 3:1-2)
Now, with the interpolation, John the Baptist assumes the role of the ocular witness of the descent of a mythical being ("the logos of god").
This means that in the original incipit of Marcion's Evangelion, the voice of god addressed Paul himself, with the words:
You are my beloved Son
Since so we read in Galatians 1:15-16:
But when God, who set me apart from my mother’s womb and called me by his grace, was pleased 16 to reveal his Son in me so that I might preach him among the Gentiles,my immediate response was not to consult any human being
This explains why the incipit of Mark 1 is read easily by Tarazi, Dykstra inter alia as midrash on Paul.
My point is that Paul, in the original (but lost) marcionite incipit, figured as the original witness of the descent of Jesus on earth.
This explains why: 15° year of Tiberius.