I should have clarified; my guess is that it's the lower housing of a 1392980, the complete internals of a Unicomp
UNI04C6 (IBM 1394054?) and upper housing either from the same UNI04C6 or from a later 84 key space saver due to the blue label, for which 1991 is too early.
With regard to detachable cables (funny, Star Wars V springs to mind...), I recently swapped the cases of Greenock 1391406 (buckler, detachable) and 71G4643 (all domes, fixed), and each fitted just fine in each other's, so keeps that possibility open, and is further evidence that it's not an original 1392980.
With regards an extra key being added by someone other than Unicomp, even knowing that it would be viable takes a special level of dedication and so seems highly unlikely for anyone other than us lot to do. And it would have been converting away from what I figured was the preferred standard US preference of a nice long left shift key.
In any case, hopefully the seller is being honest about functionality.
Indeed, pictures of the insides would be especially interesting!