They are not listed in my 2017 Scott Catalog. I found them listed in Delcampe under France Collector stamps. Scott must not list them. Does anyone know if they are listed in Y & T, Michel or one of the other European catalogs?
In the Yvert Yellier 2018 catalog there is a chapter entitled "Les feuillets collectors"
They do not have their own YT number and value (reference is made to the "Socota" number)
There is a catalog issued by "Socota" (see bottom right)
These are some kind of "personalized" stamps, printed on order with variable designs. The frame with the inscriptions "FRANCE / La Poste / {value}" is typical for these and is the constant part which helps to recognise them as postage stamps.