My bad. Simple as that. I made the mistake of assuming they were stainless steel because of their original looks.
Also, they had some kind of "blacking" applied to the surface to bring out the diamond plate and Chevy logo design ... it smeared when I checked it out -- I thought, honestly, that they put shoe polish on them!
Finally, after they had oxidized, I tried GIBBS to clean them up, but it was too late -- the GIBBS did nothing.
The biggest lesson of all that I have learned in modifying my first stovebolt is to avoid aftermarket products like the plague!
