What Is This Thing On My French Stamp ?

Hello everyone ! Merry Christmas !
What is this thing on my stamp ? Is that something about newspaper or advertisings ? Or I'm not right ? Thanks for any informations ! (Sorry for not good enough image quality, hope that's still enough clean)

CS

It's a piece of another stamp or label stuck to the back. Maybe that is why the stamp is missing a corner. Someone tried to separate them.

Thanks ! What stamp do you think it is ! I know it's a hard question but could you identify this stamp even if you only see the corner ?

Not I but I bet someone here could using the engravers lettering that are visible. How much of the other stamp is visible from the back?

You can only see this :

It is a stamp from the Sage issue used in France 1876-1900. You can search the Delcampe stamp site for " Sage " and compare your stamp with what you see there. (both the color of the print and the color of the paper)

Oh great ! Thanks for the information !

The remnant seems to stuck on the front of the stamp, not on the back. is that correct?

What do you mean by correct ?

I was asking if my observation is correct in that the remnant of a stamp is stuck on the front of the stamp and not on the back as stated in earlier responses.

Your observation is correct !

It is on the front! That changes everything.

Just drop it in some water and let the piece stuck to the front come off...

Oh thanks ! Drop the entire stamp ?

Yup, the entire stamp and just wait for a couple of minutes. It should pretty much just float free. Once the stamp is clean, take it out of the water and place it face down on a paper towel or blotting paper and wait for it to dry most of the way. it will likely start to curl a bit once it gets to that point. Then put in between two pieces of paper and weight it down with a heavy book for a few days and it will finish the drying process.

Thanks for telling me how to do !