>>>Can you post an image of the marks to the right of Rectifie C.A. & 17,1mm & 17,0mm? That might point to a maker.<<<

Raimey,
to the right of 17,1 and 17,0 are the proof marks of St. Etienne and the name, written in letters. Together with PT under crown on the left are the only genuine marks on this faked shotgun and it's not only this.
Ger