Wegman valves offers various types
This type of butterfly valve has 3 offsets. Same as the double eccentric this is the stem which is out of the valve body center and the stem being out of line with the sealing area in the body. The 3rd is the sealing area which is flat or horizontal on 1 side and is under an angle on the opposite side if you look at the 2 dimensional picture.
The seat is metal and in the body part of the valve, the sealing material is on the disc and is a laminated disc, held in place by a retainer.
The 3rd offset was discussed above in 2 dimensions but as the reality is 3 dimensions it means the angle of the seat area is different on the circumference. The laminated sealing material is being forced into shape to adjust to the seat angle.