Paolo, Yes, I have seen this before. I can tell you it is quite unusual as I have opened up hundreds of holsters over the years and have only seen one other made this way with a full liner to the toe.
As David says I showed it to him a year or so ago when he was kind enough to visit me at my Colorado home. It is on a WW1 holster and I would have to dig it out of my parts bin to tell you the maker and date.
I have only seen it on the WW1 holster but never on a WW2.
I suspect it was an oversight on a cottage Saddler's part and is not to be seen on a regular basis. The leather used on the example I have is quite thin on the actual back and it could also be the attempt to strengthen an otherwise too thin piece of leather. The only glue I have found that the Germans used on Luger holsters was employed gluing the liner to the back. All other parts were stitched onto the holster without the use of adhesives. The glue used is now very thick and stiff, like a sheet of brittle plastic.
The methods of building Luger holsters have interested me for years. Thanks for bringing up this interesting subject.
On another kindred subject, I recently opened up a clients holster for repair and found two very interesting Waffenamt ink stamps inside. I believe they are leather hide inspection proofs. As I said, I inspect many holsters inside and this is the first time I have ever seen these. Very unusual and fascinating to me. Jerry Burney