A lamp has a polygonal-section plug part extending along an axis and having at least one radially outwardly directed face formed with a radially outwardly projecting retaining bump. A socket has a core body formed with a polygonal-section socket hole complementary to the plug part and into which the plug part can be fitted axially in an installed position and that substantially completely prevents angular displacement of the plug part relative to the core body in the installed position. A sleeve surrounds the core body and can angularly shift relative thereto between a freeing end position and a blocking end position. The sleeve is formed with a radially inwardly projecting retaining formation that, in the installed position of the plug part, is angularly offset from the bump in the freeing position and that operatively engages and blocks axial displacement of the bump in the blocking position.