So...dagging like this was a thing in 1865...
|1865 Fashion Plate via the Bartos Collection|
Both the fashion plates above are from the Victoriana website and date to 1865. The lady in the blue and white dress has very exaggerated puffy daggin while the little girl and the lady on the far right of the other fashion plate have dagging that is more similar to the extant gown. The sleeve style, by this point, is a bit old fashioned but the waistline is fashionable for 1865.