Hi Eric, I don't think this backing sounds like Fortisan. I have an old bow in my collection, which I believe to be yew backed with Fortisan. The bow came to me broken(it wasn't when I bought it as part of an auction lot. Either the owner tried to draw it before shipping, or the freight forwarders, did when inspecting it for shipping over to me in Australlia).
Anyway, the backing appears as a sheet of industrial manufactured, long, straight, parallel fibres. It has the same colour as sinew backing, translucent, pale amber. I don't know if its current appearance is due to age(yellowed varnish) or whether it always looked the same). Where the break is the filaments look like strands of dental floss, just a lot stiffer, like long slivers of split bamboo, with no nodes.
The backing on Gordon's bow sounds just like the fiber backed bows in my collection, ie a flat, opaque, coloured sheet, usually black or red, sometimes white. Fiber(Vulcanized Fiber) has no apparent fibre pattern, as its made from wood pulp, like paper.