I have a 1964 1 franc coin of Réunion, uncirculated, with a design feature that I cannot find anywhere listed. The obverse is normal for the issue, and the reverse appears so at a quick first glance. But on the reverse is a tracing of the outline of the woman on the obverse. The outline is inverted, so that it matches the position and size of the design from the front, as if the viewer were looking through the coin. It is not a clashed die error, because the outline is a single raised line of uniform height and width throughout, obviously put there intentionally. This outline is only found in the field and does not overlap the normal reverse design, as if being in the background. Otherwise the coin appears normal: normal aluminum metal, standard thickness, no word "essai" or other indication of special issue.
Please show us the other side of your coin also. Edit* If it's this one please show both sides of your coin facing upwards not upside down.
Looking at the Obverse side with the head of Marianne, it seems as if the profile has an incused look to it. So if it were a clash the Reverse would show a raised outline.