"I think the logical answer is that someone (God knows why?) removed the serial number from your Krag's receiver. It should be where there is a vague 'cross like' shadow area on your receiver." Quoting BR
If someone ground and buffed the number out, there would be a major depression. A strait edge/ruler would show up such a depressed area. Lay the ruler length wise above and below the stamped area and it should show up easily.