In 1879, the old 37 1/2c green definitive was reprinted in bistre brown (as seen above) solely for the purpose of being surcharged Un Franc. This saved the expense of having one-franc clichés prepared, as there was no longer any postal need for the 37 1/2c denomination. Some unsurcharged sheets of the 37 1/2c bistre brown found their way into the collectors' market. As such, it is an unissued stamp, being neither a proof nor an error.
On the 37 1/2c stamp surcharged Un Franc is found the 'Un Pranc' error. It is considered to be one of the great rarities of Luxembourg philately, cataloging 6500 Euros mint and 7500 Euros used in Prifix 2007.
The Un Franc surcharge is believed to have been set up in a form of 50 with 5 rows of 10. The Un Pranc error occurs in positions 26 and 76 in the sheet of 100. As a quantity of 104,200 was printed, 2084 Un Pranc errors once existed. However, the Luxembourg postal authorities noticed the error very soon, after only a few sheets had been distributed. They suspended sale of the surcharged stamps, recalled all of the sheets, and removed and destroyed the errors. C S Holder FRPSL writing in Luxembourg Postal History: An Introduction speculates that "perhaps less than 20 examples were actually used."
Prifix 36a Un Pranc error (forgery)
The forgery shown here came from an accumulation of classics that I was recently checking for forgeries. I realized that it would have been very unlikely for an Un Pranc error -- released on or shortly after August 16, 1879 -- to have been used on mail canceled at Grevenmacher in 1880. And upon closer examination, the stamp is not from position 26 or 76 of the 37 1/2c printing, conclusively disqualifying the surcharge as authentic.
The characteristics of positions 26 and 76 in the 100-stamp setting are described in René Muller's 2004 book Études sur les Timbres-Poste du Grand-Duché de Luxembourg. An authentic Un Pranc error from the Luxembourg P&T Museum collection is shown in Robert Danzer's article, "The Story of the 'Un Pranc' Error of 1879" (Castellum, Vol. 3, No. 2 [September 1999]). Danzer mistakenly refers to the two key positions as "27" and "77."
So then how did this damaged one-franc stamp postmarked at Grevenmacher come to have what is a very genuine-appearing 'Un Pranc' surcharge? I believe you will see answer below:
Notice how the top arm of the 'P' has a sharp point on the right hand side. The 'F' of 'Franc' on a genuine surcharge appears to have been changed into a 'P'.
Beware of such philatelic pranks!