Handle -x => 9xxx-9x.  #4547.

Originally motivated by
https://code.bioinformatics.northwestern.edu/redmine/issues/show/9687.
(cherry picked from commit 473b0a7518a86d8489bab112f52168f337b29151)
Merge pull request #257 from NUBIC/4547-yyyy-mm-guessing-legacy

Handle -x => 9xxx-9x.  #4547. (1.6.7.legacy branch)