Wow, Philippe
That is a nice helmet. If I can get my hands on such helmets, I don't mind that it is a M91 post threaded plate LOL
Anyway, this collector whom I got my Waterloo helmets from - think he just put in all the plates without bearing in mind the model. I got off a M1867 Artillery 8 A.F with threaded posts, then M95 Oldenberg helmet stamped O.J.R. 91 with threaded original oldenberg plate and then M95 Waterloo helmet stamped J.R. 165 with original threaded posts (early plate where the waterloo bandau was soldered onto the plate). I think this guy just got too obsessed with threaded posts plates, there were no traces of the threaded posts on the liner like yours.