[Arran]

 A lot of Canadian built sets used a sheet of white asbestos under the chassis, I thought that a lot of U.S sets did too. It seems to have started around 1939 or so, either as a cost cutting measure or to conserve steel, but probably the prior since several manufacturers, like Westinghouse and Rogers, used steel plates under their chassis right into the 1942 model year. In any event it was there for a reason for it, maybe an electrical code requirement at minimum, and if it isn't falling apart I would leave it alone, perhaps painting it if it really worries you. There is a material that auto parts stores sell for making exhaust gaskets that looks very similar, but without using the forbidden "A" word as an ingredient, that one could use as a substitute, if it is not too thick. 
Regards
Arran