I'm pretty sure the original board was made from the stuff used mainly for indoors in Australia (and artists painting board) - it was softer - the outside of cheaper housing of the period was clad with "fibro" - a fibro cement sheet - when at that time contained asbestos, and masonite was used for interior walls - it was used I think externally, but had to be kept well painted - fibro didn't need to be - until the problems surfaced - when it came to be accpted as safe if painted and not disturbed.

Robin