He hit on the idea of searching through his old emails for any reference to this group. Several hits came back, but they were obviously false positives. The license plate of the pickup truck he had rented in Waco had been ZGL-4737. This had been cited on the rental contract and other paperwork, which had been automatically emailed to him. So any search for "ZGL" in an email just brought up those PDFs. But nothing else. And his email archive went back decades.