OK. Those are MCUs (multi-cellular urethane). It's common for them to harden up with age.
One trick I've heard works well is to drop them into a pot of boiling water for a few minutes. Softens them up and returns them to their original size.
Have you found a source for replacement MCUs? That would surprise me since I haven't seen any since the 90's. A lot of people swapped out the yellow foam for a coil-over shock at some point.