I've often wondered if this might be the smallest registered Luxembourg cover. It measures only 96 mm. x 60 mm. Yes, I know, some visiting cards that were sent at New Year's are smaller, but I've never seen one that was registered.
This pygmy registered cover was sent to the Ligue contre la tuberculose Loterie nationale [League for the National Tuberculosis Lottery] in Luxembourg-Ville from Rumelange on May 24, 1934. The 1.25 F postage and 1.25 F registry fee for a domestic letter are paid by two 1.25 F Charlotte (2nd series) definitives on the back.
The sender must have wanted to be sure that his or her lottery entry didn't get lost!