I have some diagrams , and they are not real clear, but it appears that the bell-shaped trim unscrews off the valve body. Proceed with caution, because it is somewhat of a miracle that you may have found stems. Any body parts will NOT be available. At some point, you will just have to suck up the fact that you are left with dinosaurs which have to be replaced.
Ingersol-Rand is still trying to cover warranty issues for the original purchaser, but even with them, I think parts other than stems are scarce to non-existent.