BR, you have the procedure down on making a one off barrel conversion, from your description I gathered you had seen several with the same look.

If you see a bunch of these in a similar configuration, with the undersized portion of the barrel in the same spots and the extractor slot with the same tool marks one gunsmith/machinist was doing a heck of a lot of them or someone was supplying gunsmiths with an easy kit barrel.
Otherwise you would see different finishes near the receiver. Pretty sure a 1903 had "fat" shoulder that butted up against the receiver. One smith would have lopped off just enough to clean up the chamber another would have lopped off 2'' and got on another part of the taper, with a different finish next to the Krag receiver.  With no published directions or a wholesaler supplying a lot of gunsmiths with the same altered barrel each would have an entirely different look. If I had a pile of barrels and did one or 2 a month, no 2 of them would have looked alike. Different tools would have been in the machine that day affecting the finish or customer requests would have made each one unique.
I kind of doubt that many gunsmiths would have had a shaper or special broach and fixture for the extractor slot. Most would have modified a woodruff cutter (like mine) or made one from scratch. A few might have made a sideways cutter and shaped them on a lathe, but that is slow and tiring process and each would have unique tool marks.
Sure would be interesting to talk to someone old enough to have seen all these things.