Linda,
I'm not sure this is a positive test since the color change is not what I'd expect. But here's another thing to try. Using 10% ammonia (household ammonia), very slightly wet a q-tip and rub very gently on an area that has not been tested yet. If you wash the piece first, do not use harsh detergents like 409, just mild soap (such as Ivory) to remove dirt.

Here's picture of the sort of color change you will get with old Bakelite. Sometimes the color is not so intense but it should be an amber yellow. I have never seen a color change that was brown - that I normally attribute to just removing dirt. Since I don't know what could be on the surface of degraded vulcanized rubber, I don't know if there will be a chemical reaction with a strong base like ammonia. If I find out anything I'll let you know.

BTW, the reason there is a color change for old Bakelite with Simichrome is that Simichrome has a lot of ammonia in it. The problem with using Simichrome is that it's a polish so it has abrasive particles in it which will remove the surface layer if you rub even a little. So for example If I polished the bead in this picture with Simichrome, I could remove the surface layer and then get no positive results afterwards.