1973 CZ400 exhaust system is made up of two sections.
There is a header pipe which fixes to the side of the cylinder and bends down under the engine, where it is spliced onto the expansion chamber by a female, male connection.
The header pipe being fixed to the cylinder receives most of the heat from the engine. This heat breaks down protective coatings like paint. No paint, no protection, hallow rust.
First check the pipe for cracks and deep rust pits.
Ji