Hello again,
Take it easy. I'm also not a native. So there is no need to apologize.
It's a pity that there aren't any markings. But the helmet type was made by a couple of the factories. For instance the JAPY , Etablissements Reflex, Jouet de Paris, Societe des Phares Auteroche, Compagnie Coloniale, Etablissements Japy, Etablissements Dupeyron, Arsenaux de Brest, Compagnie des Compteurs et Materiels d'Usines a Gaz. However , it is known that there were manufacturers who didn't applied their markings.
It may be the number of series. Or it may be the manufacturer's marking and the size 58. You may measure the circumference inside of the leather suspension webbing. The head circumference : A - 54-56cm, B – 57-59cm and C – 60-62cm.
Yes, as I have mentioned it in my post above the welding of parts was used for the early production M15. Later the kind of join was replaced with the one of two rivets.
The painting of the interior indicates the helmet was finished with the matt blue grey paint orginally. The outside painting can be different because these helmets were re-painted often, especially, after the war. Because the matt paint was introduced two years after the war started it may be be a clue that the helmet could be made in 1917.
The weight is included in the weight range for the helmet. It depended on the helmet size so it seems to be correct IMHO.
Just one more question .. could you check on the diameter of the rivets attaching the top comb to the helmet bowl? 6mm?