That may have been 305999 due to a heavy hammer strike on the stamp or a damaged stamp.  You would expect to see rifles in the 490k, and 500 to 504k serial number ranges if 505999 was part of the production run of Krags.

We are not seeing those numbers though we do see multiple krags in the 460 to low 487s that support the 489 cutoff.

Like you said we do not know his source nor do we have the rifle or a picture of the rifle's receiver to be certain. 

Of course, it could be a case of a leftover receiver that was hand stamped and built up.  However I am thinking not likely.