It looks to me like it might be a one-layer Huntley from pre-WW-I days.
Huntley Recoil Pad H & D Folsom circa 1916.jpg
JOSTAM acquired Huntley by the armistice, and it became the JOSTAM "Sponge Rubber" Pad --
JOSTAM 1916 02 Sponge-Rubber.jpg
Later an upstart competitor made a similar pad for a while called AEROCEL.