The first 30 Luxembourg official stamps appeared from 1875 to 1878 as OFFICIEL overprints on the early rouletted and perforated Coat of Arms issues. The quantities overprinted must have been very small as at the time the country's population was only a couple hundred thousand, and there was no sizeable government bureaucracy. This inference is supported by the fact that uses on cover are almost non-existent. The late Robert Danzer's collection contained only two covers using these officials. He understood the absence of covers to mean that 99% of the used examples almost certainly are forgeries. I'd agree.
These stamps frequently appear in auctions and on the Internet (often at optimistic prices), but absent a certificate from an indubitable authority such as Rene Demuth, I would doubt their authenticity. I've seen certificates signed by 'experts' of the American Philatelic Society that are, politely said, laughable. Yes, laughable because many of the forgeries can be readily discerned with the aid of only a strong magnifying glass. Nonetheless, obvious forgeries are sometimes given certificates, as in most (but not all) countries certificate signers are not held legally liable for their philatelic malpractice.
While I've seen many fine exhibits of the early Coat of Arms stamps, invariably the exhibitors ignore these bêtes noires. Postal historians can plead the virtual absence of material; traditional philatelists, however, should be downgraded when the adjudicator takes into account the challenge factor. Authentic examples of these official stamps do exist, and while they tend to be expensive and require credible expertization, they should be part of any traditional display.
During his lifetime, the venerable Bob Danzer acquired only two covers that use these early officials. You can understand therefore why I was overjoyed to recently obtain the cover shown below. Enjoy!
12 1/2c colorless roulette [Prifix 4]
Official mail from the government's historical archieves
official cachet on the back flap
Posted by H. Eltz, a government official, November 23, 1876, from Luxemburg-BHF [the railway station] to Diedenhofen [today: Thionville] in the then-German Lorraine at the 15-g letter rate, with Diedenhofen cds backstamp the same day.