This thread is quite old but still relevant, it's one of the first which appears when googling for whitening letters on lens, so here is my experience after reading the thread.
Usually I clean the barrel and rings of vintage lenses with alcohol. I use Q-tips and sometimes a tooth brush, but it's annoying because of spatters. Somebody mentioned Windex. I only use it for fungus.
Unfortunately alcohol was of no help with a lot of vintage lenses I recently bought. White numbers desperately kept their yellow-brownish cast.
blinkd_sparks wrote about "dead skin and other nasty bits".
I thought, well, it's organic dirt. Also think about what makes kitchen walls or shelves dirt: droplets cooking oil in the air, usually making a yellow-brown cast over the years. Some of my lenses look as if they were left for too long in a kitchen. And some as if they were owned by a smoker. Tobacco is another source of a yellow-brown cast.
And what do we use often for cleaning organic stain in a household? Stain remover for clothes!
We have a multi-purpose Oxiclean bottle on hand in our household. I spead the liquid carefully with a Q-tip on the characters I wanted to clean and let it work for a few minutes. I brushed lightly the dirtiest ones with a tootbrush. Then I removed it with a wet cloth, and repeated this a second time to be sure there was no deposit left.
The result: almost a miracle!
What can be the active ingredient(s)?
There is a wikipedia article about Oxyclean.
It does contain hydrogen peroxyde. I tried peroxyde alone after, with no success. It does not spread well and dissipates rapidly. Unlike peroxyde, Oxyclean shows some viscosity, spreads better and does not dissipate in the air.
I would not try it on direct metal though, but from what I find on the web, it is safe at least for stainless steel. It is said to be not as corrosive as other cleaners.
Hoping this will help somebody!