Just found this thread after replying to your wanted ad. My reply from over there:
-----
I should still have one.
Just the lens, or the whole assembly? I'm guessing just the lens, as one would need access to the trunk to take the whole thing.
And it's quite possible that it simply fell off rather than being stolen. The 78-79 Concord taillights had an inherent design flaw in that the lenses were held in place only by the adhesive properties of the seal. They were not mechanically fastened in any way. So when the seal finally gets old and dry enough, there goes the lens.
I would suggest that the owner check his driver side lens to see if it's close to letting go.