"Although the square bottle changed very little, the early labels show changes in color design and content. The twenty-one-year-old whiskey might have been a distilling experiment or a whiskey overlooked in the aging warehouse. It was therefore aged much longer than necessary. It is very rare, but since a label was printed to designate the age, there had to be at least a barrel or more bottled."
Looks like the mystery 18yo bottle is a younger relative of this one. Taken from a book commissioned by Brown-Forman in 1992 and sent to Tennessee Squires as a gift. My copy has the letter addressed to the original recipient in it.