Very interesting, but also very confusing. My collection of Carcanos has only 1 cavalry carbine, but a halfdozen of 38s. All of the 38s with butt- traps have room for the 9 inch rods, but the cav carbine has a shallower hole drilled in the stock and will absolutely not accommodate a 9 incher. Ample space for a 6 1/2 incher , though. So i don't doubt what the source says, but it's pretty hard to discount the physical evidence that directly contradicts it.
BTW, just to stir the pot a little, against all popular thinking, I think the 38 S.R. In 7.65 is probably one of the best rifles of the war!