Here is the elusive bridge-and-bar cancel on a 21g registered stampless cover, postmarked Luxembourg-Journaux,
6 Oct 1916, endorsed Postamtszeitungsstelle/Luxemburg-Stadt [Post Office Newspaper Office] at the lower left, censored at Trier, Germany, and posted to and backstamped at Berlin, 8 October 1916.
The official envelope is coded ‘No. 125. – 1916.’
The FSPL cancel catalog erroneously lists the initial date of use of this cancel as 12 July 1940.
Curiously, the only example I’ve found on a loose stamp is on the 1920 30c/25c surcharged postage due stamp (Prifix 9).