I'll toss in my vote for catalin, as well. Back in the 1950s/60s Wilkinson Sword ordered a batch of catalin grip scales for USMC officer's swords and British/Commonwealth forces general officer's swords. The started out a lovely Ivory white, but with the passage of time turned an amber/orange colour.

I also seem to recall catalin grips being commercially available in the 50s and 60s; someone with a bunch of old gun magazines may want to check out the small ads in the back...