Lavinia H. Foy Skirt Supporting Corset, patent applied for in 1863 and reissued in Fig. A is the main body piece, and the slits in it are for the bust gussets, which means the openings go at center front.
This 'Bustier' pattern fits extremely close under the bust. The bigger the gap between the two pieces shown (bottom left of the diagram) the tighter the waist will be. Basically it is the difference between the bust and the waist measurement