Flush-mounting box comprising: a body comprising: - a base; - a side wall having an opening on the side opposite said base; - an edge projecting outwards in the region of said opening; - a recess in the region of said base; said box further comprising: - a tab for clamping a partition between said edge and said tab, having a hub, said tab being delimited by a peripheral edge which is set apart from said hub and two side edges, (45, 46) each of which extends from said hub to a respective end of said peripheral edge; and - a screw with which said hub is engaged, for controlling the pivot of said tab between a position in which it is retracted into said recess and an extended position projecting externally from said body; said body further comprising means for guiding said tab from its extended position towards said edge and vice versa, said wall comprising, in the region of said base, a window opening onto the inside of said body forming said recess for the passage of said tab between said retracted position and said extended position, said window and said tab extending transversely to the axis of said screw, said window extending in this transverse direction from a first edge to a second edge opposite said first edge, on either side of said hub, the distance between said hub and said first edge being greater than the distance between said hub and said second edge, these distances being suitable for said tab to pass between said retracted position and said extended position via the window portion which is located between said screw and said first edge, characterised in that said second edge forms a stop for a respective one of said side edges (45, 46) in the retracted position and in the extended position, whereas said body is made of plastics material. a body comprising: - a base; - a side wall having an opening on the side opposite said base; - an edge projecting outwards in the region of said opening; - a recess in the region of said base; said box further comprising: - a tab for clamping a partition between said edge and said tab, having a hub, said tab being delimited by a peripheral edge which is set apart from said hub and two side edges, (45, 46) each of which extends from said hub to a respective end of said peripheral edge; and - a screw with which said hub is engaged, for controlling the pivot of said tab between a position in which it is retracted into said recess and an extended position projecting externally from said body; said body further comprising means for guiding said tab from its extended position towards said edge and vice versa, said wall comprising, in the region of said base, a window opening onto the inside of said body forming said recess for the passage of said tab between said retracted position and said extended position, said window and said tab extending transversely to the axis of said screw, said window extending in this transverse direction from a first edge to a second edge opposite said first edge, on either side of said hub, the distance between said hub and said first edge being greater than the distance between said hub and said second edge, these distances being suitable for said tab to pass between said retracted position and said extended position via the window portion which is located between said screw and said first edge, characterised in that said second edge forms a stop for a respective one of said side edges (45, 46) in the retracted position and in the extended position, whereas said body is made of plastics material. - a base; - a side wall having an opening on the side opposite said base; - an edge projecting outwards in the region of said opening; - a recess in the region of said base; - a tab for clamping a partition between said edge and said tab, having a hub, said tab being delimited by a peripheral edge which is set apart from said hub and two side edges, (45, 46) each of which extends from said hub to a respective end of said peripheral edge; and - a screw with which said hub is engaged, for controlling the pivot of said tab between a position in which it is retracted into said recess and an extended position projecting externally from said body;