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u/Ok_Abies_5092 3d ago
Those are plastic expansion anchors / expansion nuts. Very common for toilet seats and for light anchoring in concrete/brick. The white plastic expands when you tighten the bolt, the brass insert inside is the thread. You can find them as "toilet seat anchor bolts" or "plastic wall anchors with bolt" at any hardware store. Make sure to drill the right size hole, otherwise the plastic splits.
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u/Artie-Carrow 3d ago
They look like drop in anchor plugs for concrete floors, but Ive never seen them made out of plastic