I've seen a few Bakelite scales go like that - they get a kind of brown 'bloom' on them that can easily be removed with PEEK or similarand a soft cloth - but they do honk whilst being cleaned! The hard rubber ones I've seen nearly always have a raised ridge running right around the scale edge - maybe to strengthen them - and they too do tend to pong a bit if you clean them up. Both seem to come up well, though, with just a little work.