Displacement edges mask

[arenzed]

hi all guys i am new to this forum and i miss a lot of things to learn so i apologize in advance if my question has already been posted in some other thread.
In an object made up of several surfaces, I need to mask the contours to ensure that, once displacement is applied, it is tangent to the edge of a second surface and the mesh moves only in the central part (pic 1).
I tried to mask the edges of the map in ks with the contour texture (pic 2); so I put the two textures together through the color composite one as source and the other as background but none of the blending modes seem to work (pic 3); I also tried to insert it as source alpha and also clipping mask but nothing.
so I tried to insert the displacement map in the contour node as a background but not even so I got the desired result (pic 4-5-6).
Last attempt because I saw that with the gradient you can get a mask was to map the gradient with the contour and use it as a clipping mask but nothing (pic 7).
Would anyone know how to give me some advice?
thanks everyone, this blog is really amazing