It is said, and I do not know the source, that the slight depression above the lips in the part of the moustache is where the Malach presses for the Torah which was learned in the mother’s womb to be forgotten. What is the source and if true why do non jews also have this imprint?

The source that talks about a baby studying Torah in the womb and then forgetting it after the angel strikes him is Nidda 30b. No mention is made there of the impression on his lip. It is of course clear that the gemara- that talks also about a candle lit over the baby’s head in the womb- is referring not to a medical but to a metaphysical reality. The Maharal explains that Man’s soul- before it is fully attached to his physical body- is connected to an expanse of transcendent reality that is not limited by that body. That is why the gemara brings the example of a dream, in which a person, while sleeping and detached sometimes from physical concerns, can find himself in far-away places. Once a person receives at birth his form- a form that Judaism categorizes as the “medaber”(=the speaker)- he loses the ability to transcend his physical nature in the same way he did while in the womb. This is true of a gentile as well as of a Jew even though the spiritual attachment of the gentile in the womb would not be to Torah. The striking of the lip signifies that it is the union of the spirit with the body, characterized by speech, which is the source of forgetting a less physical reality.