As far as I know...No Krag bayonets were stamped with serial numbers.  Just the letters US and the date of manufacture.

Some may have special unit markings and a very few were marked with a 26, on the back of the guard, indicating their use for the experimental 26 inch barrel rifle because of the slightly larger muzzle diameter.

BTW: The Krag bayonet was patterned after the Swiss Model 1889/99 bayonet which does have a serial number.  See link below:

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