Marksc - Could you post a clear, up close, picture of your front-sight attachment and muzzle crown?

(This is always a valuable clue on carbine barrels).

Attached is a picture of two Stokes Kirk catalog pages. Some "carbines" have been made privately, outside Springfield Armory, (in the early 1900s and since), from surplus parts.

A carbine stock with no cartouche or 'circle P' stamp was likely put on outside of Springfield Armory.

IIRC - The U.S. Krag receiver went through 120 machine operations. To speed up the process, receivers were held in fixtures in pairs, so that machining operations were performed on two receivers simultaneously.

That odd 'angled' cut on the OP"s receiver looks like it could have been a mistake caused by something getting out of whack during a shaping operation???

IMHO - If something was cosmetically or dimensionally wrong with a part, during the course of Krag production, it would have been noticed with all the inspections and gauging that went on. Likely, the part would have been 'tossed into a scrap bin'. A receiver was likely "Right" before giving it a serial number and facing the prospect of paying royalties.

Some experimental Krags, like Cole and Metford rifling tests and Parkhurst Device trials, were documented by serial number.

Serial numbers are a handy thing to have on prototype, test, and trial rifles for identification and comparison purposes.

(I'm not a Machinist, but, my father sure was! I'd like to see more details on that Krag. It's an interesting puzzle).

New Picture of receiver showing non touched finish

Wish I had a receiver in hand to compare with but something is really different there. My first impression is it is not an accident, could be wrong here. Another photo with some light in those dark areas would be helpful. I am thinking I see half of a counter bore that should not be there.
Multiple cutters were mounted on the milling machines that did that work. To mill that step would have taken a lulu of a mistake if not done on purpose.
Assuming this receiver got the proper heat treatment, it should be safe to fire if everything else is normal. The sharp flat bottomed cut will weaken things but it is in a NON critical part of the receiver.
Hope a better photo will help figure out the puzzle.
Figured out what that half counter bore is, that is the pocket the trigger assembly fits in. The darkness in that area makes it hard to figure out.

Thanks for the latest photo 'Marksc'.

It looks to me like Springfield Armory workers missed some machine operations on your receiver, before it was heat-treated and Proofed. This could not be corrected after heat-treat.

I've never seen an unfinished receiver before. Cool!

That probably explains the absence of a serial number. It was probably 'scrapped', but, someone later made a "carbine" out of parts.

Attached - Contrasting photos of Mark's receiver and a model 1899 receiver, showing considerable difference in 'trigger pivot' area. The receivers should look identical.

Now see the difference you are talking about. Another Picture trying to get the angle you used on comparison.

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.