Recently on the KCA Forum, there was a thread on model 1896 carbines. There was some discussion of the model 1896 carbines that were in the hands of U.S Regular and Volunteer Troopers at "San Juan Hill".
That lead me to wonder where Frank Mallory found the serial numbers of "Rough Rider" carbines. (The number of 'known' RR carbines is only a fraction of the 1,000 or so issued to the Rough Riders).

I kind of found "Frank's answer", last night, on The Springfield Armory Site. It was in foot note information under a model 1896 carbine in the SA collection.

Here is part of the foot note. (I did not copy the list of RR carbines & Troopers, which is commonly available, elsewhere).