source: De Gracieuse archives, year 1872
Basically not sewing the seams together and then felling them, but by first folding part of the seam allowance up on the underside and down on the top side, then pinning the layers together and then sewing them down at the edge of both folds. The gussets are inserted in a similar manner as shown on the top illustration.
The pattern is from 1876. I found it from one of my favorite resources, De Gracieuse archives. It's the bottom left corset here and the pattern pieces can be found here.
It also has several additional bone casings made by sewing strips of coutil inside the corset. The flossing is made with graphite grey silk. It's boned with spiral steel.
For the first time I'm happy with the shape with no complaints. It looks quite small waisted, but in fact it isn't any smaller than any of my previous corsets. It's just an illusion created by having enough flare for bust and hips.
And a few close-ups: