I have rejuvenated many of the black Delrin wing nuts by gently scraping off the haze with my finger nail or dull blade/point and then applied a glycerin-rich leather cleaner (Lexol). It worked on a "Criterium" RD too. I rub in the cleaner with my fingers or a Q-tip.
Before and after photos here: http://s517.photobucket.com/albums/u335/gypsybike/RESEARCH%20MATERIALS/?action=view¤t=000aahaze.jpg
It has held up very well.
Jay " the alchemist" Dubiel
Halifax, VA USA