Don,
As 1967dartgt suggested, it's a hard anodized coating to protect the piston for nitrous use.

I've spent the last two evenings working on the 1 Ton to be ready for the trip, so not much time for this thread.

When M/T started changing their ET Street tires this year, I decided to order a bias ply set for the Valiant ahead of time. I also decided to move up from my 28 X 13.50s to the 31 x 14.50s that both Dale and Boone use.
Finally got around to having them put on the wheels Monday, then tried to mount them on the car...classic case of my eyes being bigger than the wheel well opening!! 30.5 tall, sure! 31, no not even! Back to the drawing board, and if I wanted the 28 X 13.50 that I had previously...they are on back order at Summit until 9/29!