part of a sheet metal housing to a structural support the exemplary fixture disclosed is a lighting fixture attached to a wiring conduit the mounting plate has a generally circular mounting aperture a twist lock bushing includes a tubular body correspondingly generally circular in cross section the mounting plate and the twist lock bushing releasably engage each other by relative movement to insert the tubular body into the mounting aperture with the mounting plate and the bushing angularly oriented in an insertion and removal position with reference to each other followed by rotation of the mounting plate and the twist lock bushing relative to each other to an installed position application_number US 72881500 A description referring first to fig1 a representative lighting fixture 20 includes an upper housing 22 made generally of sheet metal a lower housing 24 containing a relatively heavy ballast transformer and a socket 26 for a high intensity discharge lamp not shown the fixture 20 typically has additional elements not shown in fig1 such as a reflector and a lens the upper housing 22 includes an upper mounting plate 28 made of sheet metal the mounting plate 28 has an exterior surface 30 and an opposed interior surface 32 separated by a mounting plate thickness a suitable mounting plate thickness for a fixture weighing 23 pounds 10 kg is 0 05 inch 1 27 mm the upper housing 22 also has an end cover 34 which is removable to provide wiring access.