Paris Mystery Mark

Back in Vol. 19, No. 1 of the CCSG Bulletin I raised the issue of the DD machine mark on mail transiting France to the Red Cross (et al.) in early 1944. Since then Chuck LaBlonde has mentioned and illustrated it (and others of the same design with different letter combinations) in his mail to/from Switzerland books (naming it the "Paris mystery mark"), and Horst Landsmann has illustrated it in his WWII German civil censorship book, noting that it's relationship to the German censor station inn Paris is unknown.
It would be great to hear from anyone who has solved the mysteries of who applied these markings and why. And even if you don't have those answers but do have additional examples of this sort of marking, it would be good to have scans of them. Perhaps a pattern suggesting the answers will emerge.
It would be great to hear from anyone who has solved the mysteries of who applied these markings and why. And even if you don't have those answers but do have additional examples of this sort of marking, it would be good to have scans of them. Perhaps a pattern suggesting the answers will emerge.