Regency Seams

Looking closely at the side back and shoulder seams you can see that they have not been sewn together in the manner you might see 18th century garments, particularly frock coats. In these garments you see a two layers (fashion fabric and lining) sewn face to face with a piece of fashion fabric, then the seam is ironed towards the single piece of fabric. Over this seam and single piece of fabric is laid a piece of lining, the seams tucked under and then whip-stitched to the existing seam.
(After Christmas I will put up an example)

In this gown it looks like a lining piece has been attached to each pattern piece, whip stitched so that there is a small amount of fashion fabric showing on the inside. Two pieces are then sewn together so that a ridge forms, and is shown at the side and shoulder seams in this photo. I cannot open the garment up to confirm this theory, but on close inspection it looks very probable.