Upon doing more research, Haartz's website glossary and their
Vintage Restorer's Guide should shed light into what the original fabric layer may have been. I have not had the time to fully read their information about period materials but it is quite interesting. Note: besides the glossary link posted, their Guide has other linked chapters.
http://www.haartz.com/vintage-restorers-guide/glossary-z
Thank you for Glenn
@Glenn Rhein for the images. Haartz also addresses multi-ply materials on this link:
http://www.haartz.com/vintage-restorers-guide/general-terms-types
Interesting, in all the Silver King literature the seats are described as full leather covering for both the Toolbox and Steamline models.
In the book Scott M. sells: Vintage Bicycle Trinkets 1932-1942 the Troxel illustrates a toolbox saddle that appears to be a fabric material, dark with light colored trim. In the ad (August 1935) it is mentioned "furnished in any color;" has anyone seen this seat in other colors besides black?
Should someone be inspired to do restoration or reproductions this Haartz info would be a good starting point.