More pics of the krags I asked about...
Sorry, I had a hard time with multiple pics.
The attached is a word doc.
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Based on what I saw in the full sized pics in the first post, I'd say both were cut down rifles rather than true carbines.  The longer of the two definitely has a replacement front sight...  Just my opinion...

kaziboku:  Your Krags definitely appear to be cut-down rifles.  Since the originality is gone, it makes them great "guilt-free" shooters.  None of your posted pictures show a clear view of the receiver markings, so it is hard to give feedback or information.  I have never encountered a U.S. Krag without at least a damaged serial #.  But, I suppose anything is possible in a mass produced item. The serial number marking is normally about the same depth as the "U.S." and Springfield & Model Markings.  Given the surface hardness of the Krag action, it is unlikely the serial number would be removed by excessive polishing.  It would require grinding, machining, or welding to remove all evidence of a serial number.  In my opinion, discoloration and disruption in the metal surface of the receiver should be noticeable.  A clear side view picture would be helpful.

Another reciever pic - Thanks for the inputs.
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try taking the photo in a dimmer room without a flash in "Night Mode" you may be able to distinguish the serial, or go to a Military Surplus store get what is called sight black spray some on the area where the serial number is, use a small block of wood and an old t shirt, pull the t shirt tightly around the block and rub across the sprayed area, if any concave areas exists the sight black will stay within the crevices, revealing what is left of the serial number. Or maybe there was an error in the stamping of that particular rifle and it was never stamped, If in fact it was sanded off there would be alot, and I do mean alot of metal missing. making it an oval or egg shape. im sorry I cant help more. hope it works.

To see if its been sanded, take the bolt out and look down the rifle at a sharper angle, if there is any metal missing, you will be able to tell. it should be perfectly round. anything less means its been tampered with.

Removed the bolt and the reciever looks pretty round to me.  Perhaps whomever did the sanding was really good?