According to the wikipedia site Rexine was also used as a bookbinding material.
On the Dutch "Marktplaats" site there is currently a huge amount of Rexine for sale.
The seller calls the material "Bookbinding linen Rexine".
€50,- per roll 25 meter long, 1,3 meter wide.
Could this be the same material as used for the saddle covers?