I took a break from doing eyelets today to try out a few seams to see if I could replicate whatever is used on the mystery seam of the sacque.
First: the running stitch.
I can't see the stitches at all once the upper layer of fabric has been folded back, so I doubt this is it.
Second: slip stitch.
I can see the stitches, but they're very slanted - you can tell which direction each stitch is going in (up vs. down, I mean).
I think this is it! The stitches are identical, as they're all going in the same direction (around), and highly visible. I also found it quicker than the slipstitch, though that might be my imagination.
What do you think?