JMH,
Again, see my discussion in your previous thread about oil in vacuum lines. After thinking about it, I thought it would be a good idea to have an extra diaphragm in case my new one should wear out. I went to my local Chevy dealer (the place I detailed in the other thread), gave them part # 3847058 and they printed two pages of dealers across the U.S. who had the part on the shelf. They ordered me one, $49, and it will be here in two days. It is the correct part number.
Although obsolete, they believe there is some more modern application to this diaphragm or it wouldn't be listed.
If your dealer is not able to help you, call Chapman Chevy in Tempe, AZ and ask for the parts dept.
Kent