Assembly made up of a panel and a device for fastening a fitting to a face, known as the main attachment face of the panel, comprising at least one mortise that passes through the panel and opens out onto said attachment face, and to a face, known as the main opposite face that lies on the opposite side from the attachment face, the fastening device having a longitudinal axis and the assembly being characterized in that it comprises: - a clamping member comprising a jaw, known as the clamping jaw, which is shaped to be able to pass through the mortise translationally along the longitudinal axis and to be able to bear against the opposite face following rotation about the longitudinal axis and comprising a cylindrical rod having means for attaching the fitting to the attachment face; and - a peg for anchoring the fastening device in the panel, said peg rotating as one with the clamping member about the longitudinal axis of the fastening device and comprising a jaw, known as the anchoring jaw, which is designed to be able to bear against the attachment face, and an error-proofing feature for guiding the anchoring peg into the mortise, rotating as one with the anchoring jaw and facing the clamping jaw, said error-proofing feature having a hole that coaxially extends the anchoring jaw and is designed to receive the cylindrical rod of the clamping member, and an external envelope shaped so as to engage with the internal surface of the mortise in the panel in order to prevent the fastening device from rotating in said mortise; said clamping member being designed to be able to be moved translationally with respect to the anchoring peg along the longitudinal axis between: - a position, known as the blocking position, of the fastening device, in which the clamping jaw and the anchoring jaw have been moved toward one another by elastic return means and are in contact with the two main faces of the panel; and - a position, known as the installation position, of the fastening device, in which the clamping jaw and the anchoring jaw have been spaced apart from one another with respect to the blocking position so as to allow the clamping jaw of the fastening device to rotate about the longitudinal axis. - a clamping member comprising a jaw, known as the clamping jaw, which is shaped to be able to pass through the mortise translationally along the longitudinal axis and to be able to bear against the opposite face following rotation about the longitudinal axis and comprising a cylindrical rod having means for attaching the fitting to the attachment face; and - a peg for anchoring the fastening device in the panel, said peg rotating as one with the clamping member about the longitudinal axis of the fastening device and comprising a jaw, known as the anchoring jaw, which is designed to be able to bear against the attachment face, and an error-proofing feature for guiding the anchoring peg into the mortise, rotating as one with the anchoring jaw and facing the clamping jaw, said error-proofing feature having a hole that coaxially extends the anchoring jaw and is designed to receive the cylindrical rod of the clamping member, and an external envelope shaped so as to engage with the internal surface of the mortise in the panel in order to prevent the fastening device from rotating in said mortise; - a position, known as the blocking position, of the fastening device, in which the clamping jaw and the anchoring jaw have been moved toward one another by elastic return means and are in contact with the two main faces of the panel; and - a position, known as the installation position, of the fastening device, in which the clamping jaw and the anchoring jaw have been spaced apart from one another with respect to the blocking position so as to allow the clamping jaw of the fastening device to rotate about the longitudinal axis.