The original number is from December 1917 production, but the new number could be matched to a wide range of dates and makers. I would say that this barrel is on at least its third rifle and may have been better than the 1923 barrel it replaced. It has not been matched to its present rifle and there is no fitting date that I can see. Still, I have seen similar direct from war stock. D.
It was certainly a very different time to todays 'throw away society'.
Rear sight used in service on 3 rifles, now on its 4th (my build)