Well, you may have figured this out by now, but I'm pretty sure that the difference is that the ones that were built prior to 1898 are considered "antiques" and as such not classified as firearms under US federal law. The ones manufactured 1898 or later are considered firearms, but are still considered C&R. (IANAL, etc)Also, pp43-44 list several 15in 1894s as removed from NFA as antiques, rather than C&R, though the exact difference there escapes me at the moment.
We have at least one BATFE member here who might be able to help you figure this all out.