67 Fastback rear deck filler panel repair ?s

Yes it was a leaded factory seam. Here's a picture of a NOS quarter laid onto a brown car, left side shown. This lead joint was melted out and spot welds drilled.
You can see that after the 2 panels were spot welded together, a channel was formed to receive the lead.

It is common for moisture to migrate into overlapped panels causing rust through.

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