Sorry don't have a 68 Dearborn example to offer so I added in a 67 example and since we find 69's done in the same similar fashion IMHO its likely that they did the same in 68. One additional note is that on some the painters didn't apply allot of paint to the bottom surface of the door. In those cases the sharp edge is not seen but instead a misting of either the interior color, exterior color or both showing no real edge and often after this amount of time - surface rust
68 San Jose examples
68 NJ examples
67 Dearborn example
The the hand applied bottom tape line was likely attached to a precut mask (don't know if this was a reusable or paper throw away mask) Because of that there can be allot of variations. The pictures show what appears to be a single piece of tape or a bunch of smaller pieces slapped in place
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For a extra bonus detail - don't forget to black out the defroster vent area of the top of the dash or you might be sorry