what sealant did the factory use?

Window weld urethane, it sticks to glass, it sticks to rubber, it is black.

Can go around the window rubber with a fine bead. Wear some mechanics Gloves, hard to get off your skin.

Can smooth It in with a wet finger quick before it sets. Can paint after cured if you need to.

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Went around my whole '76 D100 windshield to fill in the poor fitting gaps and seal it from any water getting in.