Great pictures, @caryg!
@christinel23302, if I might add, from the second picture if you zoom in on it, it appears there's a metal screw holding the white keeper onto the shaft of the mushroom. I'm guessing that screw worked its way out after a number of years and is now mixed in with the grounds you probably threw away.
Anyway, the spring goes over the mushroom and the two pieces together get inserted into the bottom of the filter holder. Then the white piece (white in the second picture, black in the first) goes inside the filter holder and is supposed to be held on with a screw. Assuming the screw has vanished into that great recycling bin in the sky, I see no reason that screw is particularly special; it may be zinc coated as protection against corrosion, but I would imagine any zinc or stainless steel screw that fits the hole would work as a replacement. Your local hardware store should have something suitable for under a buck.
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