Yal:

The strut stop is a rubber piece that fits on top of the strut hydraulic arm, brick in color, and protects the strut casing if you happen to fully bottom out the strut arm.

I would recommend replacing them as well, as you'll see when you remove the strut it probably has deteriorated quite a bit. They tend to flake too as they age.

Good luck!

~Paul

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'91 300E, 207k miles