The November 1971 Kickshaws introduced the problem of finding thirteen four-letter words which together contain each of the 26 letters twice. Leslie Card exhibited a list of fourteen four-letter words, with four extraneous letters. Dave Silverman commented "we don't expect anyone to come up with a 13-word list in the near future" -- and he was right.
"Double Pangram Lists,"
Word Ways: Vol. 11
, Article 17.
Available at: http://digitalcommons.butler.edu/wordways/vol11/iss4/17