Nothing strange about it.  The 1898 carbines were already in progress when a change was made to drop the saddle ring and increase the commonality between the rifles and carbines.  1896 carbines were given "1899ish" stocks during rebuild - as were the 1898 carbines.

Whereas the 1898c and 1899c share the same receiver, and thus stock, the 1896 carbines have a different bolt well - requiring a slightly modified stock to include the scallop.

Not "rare" and not "valuable" as people prefer 1896 carbines in the earlier saddle ring stock.