An assembly, counterweighted and/or motorized, to which drops or flats can be temporarily affixed while getting their finish treatment. The entire apparatus raises and lowers through a slot in the floor. Once a staple of almost every scene shop, have fallen out of favor as scenery has become more 3D, and less reliant on the detailed, painterly finishes of "wing and drop" and "box" sets. These days, most scenic artists prefer to paint drops and flats "down," i.e. laying on the floor. The drops that is, not the painters. Except after an all-nighter just before final dress.
http://www.qc.cuny.edu/Academics/Degrees/DAH/DramaTheatreDance/PublishingImages/paintroom3.jpg
http://www.qc.cuny.edu/Academics/Degrees/DAH/DramaTheatreDance/PublishingImages/paintroom3.jpg