Hi-Ten 1020 tubing. the dealer would have sold the fenders and the kickstand as an option when the bike was new. I owned a red '72. This is about the same vintage. I don't remember exactly,but the seatpost was very small, as the tubing is pretty thick. Probably a 25.4 would be a good place to start fitting.
It was a great rider, far beyond its materials. I rode it for ten years...