Argo,
This Halifax is clearly marked with 6.5 for the chambers, which, would tend to indicate construction prior to 1912 or so, based on what we guess about the use of that mark. The proof level, at single proof, would be my guess where Fab is taking his stab at the age, as the laws changed in 1924. See if you can clarify how he is drawing that conclusion on the date of the guns manufacture.

My French sucks. It comes back, a bit, when I use it, but I miss things in reading.

Best,
Ted