Argo,
To begin with, it is Ted, not Tim.

While it is easy to say a Didier barreled gun has a date stamp, it is a much different thing to document that. Further, even if it is a date stamp, what would it represent? It could be the date the barrels were provisionally proofed, the date the barrels were assembled, or the date the gun was final proofed. We really have no way of knowing for sure. There could be YEARS between any of those events, and when the gun reached inventory, and years between when that happened, and when it was sold.

Picture number 6 is a gun I am very familiar with, as it lives in my neighborhood, and I posted a bit about this gun here on the board. It is the wreakage of a high grade Darne that doesn't fit the description of R, V, or P model. The picture you posted is missing the proof stamp, which, was for powder J. 1900 was the year powder T was introduced, but, for at least a few years after that, one could specify powder M, J, S, or T. Hence, we can't even state with authority when a gun was built based on the powder it was proofed with, save the fact that anything proofed with powder T would be post 1900. Further confusing what and when with this gun, is the fact that Regis Darne's offerings were pretty well sorted by 1900, and this gun doesn't appear to be a cataloged model, or, marked with a stamp in reference to grade.

Documenting an exact time of change for the markings 6.5 and 65 could be problematic as well. The proof house in St. Etienne is a literal hive of activity, and the day I was there, there were rack upon rack of modern pump and autoloading barrels being proofed for Verney Carron, among all the other barrels. It would seem to me that the 6.5 mark could have very well been used alongside the 65 mark until it simply wore out and was discarded.They mean the exact same thing. Again, documentation would be wonderful, but, we simply don't have that, yet, and anything else is speculation until we do.
I was very lucky on the day that I visited, as if there had been military contract barrels of any sort in the building, access would not have been granted to me. I would be willing to bet few Americans have toured the facility.

Best,
Ted