What is a GM bellows? Hope you don't mean N.O.S.
We must be careful with the word N.O.S. It always seems exciting to find something never used. It is exciting for parts that are not made of, or contain, plastic or rubber. I would not recommend buying plastic or rubber parts that are already 30-70 years old for our trucks. Chances are that the rubber has lost all of it's properties and will fail in short order.
Parts with N.O.S. springs, gaskets, o-rings, seals and diaphragms would be on my no-no list also unless these would be rebuilt with actual new innards.
The above also applies to "used" parts of course.
