John:

I recently sold one of those 1899 carbines (modified for gun sling) that I had for about 30 years.  With the carbine, I received the Rock Island 1902 gun sling (stamped as such) so I was reasonably sure the carbine was a Ft. Leavenworth modified carbine.  This 1902 sling was made specifically for this mod.  Untill someone comes out with a more definitive work about these modified carbines, I don't think I'd pay any premium for them.

Carl D'Agnese