"Dear Sirs
This morning Mr. AUER Hermann, very talented Master Weapon in Brunico (BZ), Italy, of which I am honored to own a Bolt and that "imposes" more often on arms issues, told me that of Test Benches, in the Austro Empire - Ungarico, there were several and in many places. This pushed me to further research on the Internet and I happened on the website of Dr. Edoardo Mori that I report here:
https://www.earmi.it/armi/database/Guida...0da%20fuoco.pdf
from a quick search, I seemed to understand that the punching of the crown with N below and the initials St.m.G should refer to Germany, period 1891-1912.
I did not find any elements on the abbreviation B.U followed by serial number.
I think I will contact Dr. Mori as soon as possible."

You should find somewhere book Speed-Schmid-Herrman: Original Oberndorf Sporting Rifles
There is a long list dated m98 Mauser serial numbers with BU, BUG and BUGN proof markings. With this list I dated yours rifle.