Lloyd....don't know for sure about the '69 but my original '68 Z28 has two different manufacturers for the exhaust manifold bolts. One side had the raised circle bolts with a "RSC" inside the circle. The other side had a flatter circle on the head with a squiggly "X" inside the circle. My computer that has my camera software on it is down right now so I can't send a photo at present. Will try later tonight when I get it back up, that is if your interested.
Yes, I'm interested Jerry, please post photos when you can. I would like to see examples of head markings from a few known original cars as well determine if the photos of the bolts I posted are correct.