I do not have Brophy's book, this was taken off the web, I searched for Krag serial numbers and it came up with a whole listing taken right out of his book. Wish I did have a copy as it would make the research allot easier. I have no idea what all the differences are between the different 1898 series carbines, seems odd that there are so many series.  Knowing how or how not our government's procurement system works  leads me to believe that there was a greater need for the carbines than originally thought. And they kept adjusting production to take care of the added requests. Mounted troops in our other possessions needed carbines too, areas such as Guam, not just the Philippines. The P. I . carbines I think were for the P.I. army irregulars who were being trained and not for the regular army folks. And all of this makes sounds correct to me as the troops knew the Krag was on the way out and the 1903 Springfield was coming in as a replacement but no carbines wee to be made. So, I would think that every depot supply officer was ordering more just knowing that no more would be made. These were still in service in the Philippines until just after WW2. Some of the guerrillas fighter used the krag carbines when they could get nothing else. I have a picture of one of my uncles holding a krag carbine during the war, he was with the Fertigs raiders group after they broke him out of a prison camp.
Mike