'Bill Rodgers': What source "specifically states" that your model 1898 Krag rifle, serial number 153798, was built in 1899?

Krag receivers were numbered prior to being assembled into complete arms. Krags were not built, accepted, or issued in numerical order. Some estimates would place your rifle's assembly around November, 1898.

Some Krags close to your rifle's serial number were issued to 34th and 35th U.S. Vol. Inf. in August, 1899. (They would have been built and received acceptance 'cartouche' before being issued).

IMHO - It is unlikely a stock with a 1900 'cartouche'  was the original stock on your Krag. More likely, it was switched during refurbishment work.

That doesn't make your Krag 'wrong', just likely 'rebuilt' (many were).

It would be fun to see some pictures!