This thing?
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If you've got a fold down seat car it wasn't "missing" the divider board. It never had one. Kinda defeats the purpose of having a fold down rear seat.
On a standard rear seat car the board hangs off the seat hooks and rests on the cross brace, you can see it here from the back
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Where it would be from the front, with the slots hanging on those hooks
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But a fold down car doesn't have a divider like that
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Instead you car should look like this with everything folded down (seat and security panel)
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The "security panel" from the rear, folded down
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And folded up. It's a wooden panel that was like 1/4" plywood with carpet on one side and the latches with feet on the other
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