In the November 1978 Word Ways, I introduced the concept of symmetric crash groups - sets of 2k words of 2k-1 letters apiece, in which each pair of words has exactly one crash (same letter in the same position, as E in rovEr and cruEt) and all letters in the set participate in crashes.
Eckler, A. Ross
"The Longest N-Crash Word Lists,"
Word Ways: Vol. 49
, Article 21.
Available at: http://digitalcommons.butler.edu/wordways/vol49/iss2/21