Ted, I think we can do better than 1900-1912. Is that the Halifax 3 you pictured previously? What's the serial number (can't make it out from the "dating early 20th Century Shotguns" line). But if it has an "A" prefix, per the SN chart posted by GLS (with all its imperfections) - it figure it to be 1910. Here's the analysis"
The "letter" series began in 1910. If your gun has a letter series SN and the cm chamber mark, it would date the Halifax 3 between 1910 and 1912.
We've posted Darnes in the "C600" series with both cm and mm stamps - meaning they almost certainly were mid 1912 when the chamber stamps changed:
-- C692, a 12 gauge with a chamber stamp 6.5".
-- C643, a 20 gauge with a chamber stamp "70".
Analysis continued:
-- 1910-1912 - A001 to C650 = 2650 guns (more or less) made with chambers in cm...beginning with A0001 - ending about C650 or so)....
-- Since we don't know when in 1910 the A series began or exactly what month in 1912 the cm/mm changeover took place - say anywhere from 20 to 30 months of production = take a stab - if 24 months (June 1910-June 1912) then about 110 a month? Pretty easy then to date the Halifax within 3 or 4 months if the chart is true and yours has an alphabet prefix).
Oh yes, in Spring 1971 I drove across France in a Citroen Deux Chevaux; I owned an Oldsmobile once but in Brussels 1985-88 drove a fuel injected Renault 21, a pocket rocket - ran a BMW out of gas at 130+mph running up from Luxembourg to Brussels..
