Jean Joseph Coninx — or Jan J., as his gravestone says — has been found. He was born in 1902 in Montegnée. So not 1884 (according to some websites) or 1892 (according to his gravestone).
How did we figure this out? The old Halen cemetery list notes that he didn’t serve during the First World War — a hint that his birth date wasn’t correct. A broader search for his birth certificate leads to the correct birth date.
His parents moved from Montegnée to Herstal. He died on his birthday, possibly one of the many flu victims. After a temporary grave in Hasselt, he got his final resting place at Halen military cemetery.
Why he wasn’t moved closer to Liège, near his parents, remains a mystery.
We now know who he was. The War Dead Register has been updated. Thanks for all the tips we received!