Courthouse News Service has the details. The sale took place in 1994.
UPDATE: Nicholas O'Donnell comments: "The most obvious challenge to this case is the passage of time. ... If the painting is not a Rockwell, then a threshold question will be when the plaintiffs could have determined that, and whether it was within the statute of limitations. The 2013 appraisal quoted by the Complaint could cut both ways; if the Rockwell signature was as obviously a forgery as that appraisal believes, then the question will be why the plaintiff did not discover it sooner."