I purchased discontinued Scultured Daisy stoneware pieces in the Poppy
Trail line made by Metlox Pottery of California at a thrift store. 
(These dishes were popular in the 1960s and are now collector pieces.)
 The price was marked on the center white portion of the plate in
grease pencil.  How do I remove the grease pencil?  I have washed
these pieces multiple times in the dishwasher.  I have scrubbed them
with Mr. Clean Magic Eraser and steel wool.  I have soaked them in
hydrogen peroxide (hairdresser strength).   What else can I do?

Just a suggestion... try some WD40 on the stains.

Hydrochloric acid (aka muriatic acid) is sometimes used to clean
masonry. I can't attest to what it might do to the glaze though. You
might try diluting that with some distilled water and giving it a
shot. Let me know if it works. Word of warning though: use protective
gear and don't breathe it. It's bad stuff.

tutuzdad-ga

I'd try some Citrusolv degreaser, or a similar citrus oil based
degreaser, such as GooGone.