Leigh Mercer wrote, November 6, 1968: "A 6 by 10 rectangle can be built from the twelve pentominoes (the twelve ways five squares can be put together) in 2,339 different ways. I have drawings of these, and made a card-index of 2,339 cards from the drawings, discovering some interesting things no one else knew. They wouldn't do for WORD WAYS, I fear ..."
Eckler, A. Ross
"A Near Solution to Mercer's Pentomino Poser,"
Word Ways: Vol. 2
, Article 12.
Available at: https://digitalcommons.butler.edu/wordways/vol2/iss2/12