Today I was studying a group of covers franked with the 1916-1924 surcharges. And it's when you study material you've acquired, perhaps long ago, that you often make new discoveries.
Here is a cover that initially interested me because the 17½c 20g letter rate and 25c registry fee to Germany were paid entirely with 1916-1924 surcharges. But then I noticed something I hadn't seen before -- a curious single line strike reading 'Mondorf-les-bains' next to the registry mark. There was also the Mondorf-les-Bains cds of December 1, 1919 (and a Luxembourg-Ville transit backstamp of the same date). So what purpose did the single line mark (with "Bains" strangely spelled with a lower-case "b") serve?
The strike remains a mystery for me. Two numbers are written on the cover. 'No. 4' appears in black ink above the single line mark. And above the registry mark in purple pencil '574' is written, which I assume is the registration number.
Also, the cover is addressed to a philatelic organization (I think) in Ludwigslust (Mecklenburg), Germany. Did the sender apply the single line mark before registering the cover? And if so, why?
I occasionally see stamps canceled with a single-line device, presumably because the proper canceler was unavailable, but here the device was not used to cancel the stamps.
Got any clues?