That's the original broken hub. It's pretty well cracked, which isn't surprising since this is a 25 year old filter. Most of the plastic inside is pretty brittle and worn.

Here's the old hub, now broken in half, attached to the piping assembly. It turns out that the piping assembly spins off, which makes taking it apart a helluva lot easier.
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The black piece is the new hub. I'm going to hack off the old hub (it's glued on) and put a male thread adapter on it so I can screw on the new hub.

Here's a groove in the lateral threads. Sand must have been coming through this groove.
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The two piping assemblies thread onto these male threads. It took a while to figure that out.
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