Yesterday, in passing, I posted a link to a corset:
When I went into the library to look for a book on walking sticks, I wandered in the fashion history section and noticed Corsets: A Visual History, which features pages and pages of corset advertisements. There was one in a National (that's the brand) ad from 1912 that looked exactly like this one, with the same odd overlap in the back - it was described as a "bust-forming" corset for slight figures. So my inclusion of it with supportive brassieres - and the inclusion of the brassieres that look similar to this - was probably a mistake, as the boning over the chest is just meant to provide a hollow shape for the dress to drape over.
ETA: I went back into the book and found the actual Sahlin ad with this actual corset from 1914.