After looking at this more closely, I am fairly certain that this body was made up initially as a Swiss waist, and then converted – in period, apparently. Various styles of corselets were popular in the 1860s, meant to be worn as a sort of cross between a belt and a vest over white muslin bodies or even over dresses. Some were more like laced belts, others had straps, but almost all had deep center points. They were often made with lacing at both front and back, and you can easily see how extra material was added at the top to your example. This explains the EXTREMELY unusual fact that the lacing does not go all the way up the back – I don’t think I’ve ever seen an 1860s body that could not be opened entirely at either the front or the back.