Measured between the seams towards the end of the protruding section as per the pictures:
Leather sports = 328mm
Cloth SE = 335mm
Leather SE = 350mm
Measurements confirmed identical on both seats of each pair.
Both leather measurements could be slightly short as the seams were set a little further down the sides of the protrusion than on the cloth ones
The one that surprises me is the leather SE. I wonder if they changed the width of the seat foam later in the production run - the cloth ones are from an early-ish car ('51 I think), the leather ones are definitely from a '53.