Hydroflux valve as in welding? I tried to look up hydroflux valves on google images and went through pages and pages and didn't find anything that looked like it would have had something like that on there. However, judging by some of the valve designs, I think you're on the tright track.

As far as the other pieces, correct me if im wrong, but wouldn't the rivets be hollow inside and not solid? And I am probably wrong on the material. Its just so odd to find 4 of them. 2long, 2 short, in an ashtray with a bunch of other small nuts/bolts. But then again, who the hell knows what happened since the car was last registered in the 70s.