Certainly looks French to me, similar to the lebel bayonet frog, although it doesn't have a buckle.
but unfortunately, absolutly unknown in France
so which country ? mystery
I believe Spitace41 is correct. According to Anthony Carterís Bayonet Belt Frogs (on page 60, in the section covering French Frogs) ďDuring the 1914-18 War many damaged French belt frogs were repaired by stitching new front sections in place. In order to simplify the repairs buckles were not fitted since it was found that a slot cut to allow a tight fitting frog to fit over the Model 1874 scabbardís loop was an adequate way of carrying the bayonets.Ē Pictured below are two examples, from Carterís Book.
While there are no apparent indications your posted frog was a repaired older model and no traditional rivets were used in its construction, I believe itís probably an effort to construct a functional frog under early wartime (WWI) economic conditions. Iíd recommend you attempt to see (carefully so as not to destroy the leather) if a Grasí Bayonet Scabbard fits in there. If thatís not snug enough, then perhaps a Berthierís scabbard would fit better.
Either case, the frogís shape looks like no other countriesí style of frog except for Franceís. Carterís pictured (line drawings actually) examples of his modified frogs look similar to yours, but have the aforementioned rivets, which reinforce the fogís belt loopís stitching. I look forward to what our other collectors have to say.