As Philippe has said, this is a humble opinion offered as an
observation by collectors in France, who are quite knowledgeable.
So it is an opinion,
not a factual way to date your M91 post.
You will find absolutely untouched M91 Pickelhaube, with solid rear spine, rotatable neck vents, screw post on the Wappen, in other words, textbook M91 that do not have a large center dome on the M91 post. So perhaps it is an evolutionary issue, where some came out with large, and some came out with small domes, and by 1895 it has standardized to the smaller dome.
For example, here is an M91 that is untouched, dated 1891. the M91 post is very large thick, and long. Quite different from the "standard" M91 post. But it has the smaller dome.
More photos of the helmet
HERE