Don't know if I should broach this subject or not... but Mark stated that it was definitely a carbine.

Now here are the thought running around in my head:
The receiver is marked 1898.  True 1898 carbines are rare, and command a mighty high dollar figure.  There are many fakes running around, and the serial number is the only true way to discern a REAL 1898 carbine from a fake or simulated one.  Being that this has no serial number makes me suspicious that it is not a true 1898 carbine.  I'm leaning more towards a botch or unfinished receiver that was built into a Bannerman or Stokes Kirk "Parts" gun.  Now it may or may not have a true carbine barrel (let's face it, anyone could buy a carbine barrel and install on a rifle receiver)... but that does not make it a true carbine.

It is... however, one heck of a mystery!!!  Either way, Krags are meant to be enjoyed!  Yours just has a little additional story to tell!

Just out of curiosity, have you ever fired it? 

FWIW, a serial number-less Krag showed up here before, and it turned out that the number WAS machined off.