Of these very late 96 krag rifles that were made in 1898, have they been observed as having a 1898 Cartouche? or did they use up 1897 stocks. I haven't seen a 98 dated 96 stock.

My GUESS is that, since they were involved in the royalty program, they might have had to keep more accurate records and pay closer attention to such matters than was the case with the trapdoor where a year's tolerance (only in the "right" direction of course) is winked at in some cases.

(My logic & opinion, FWIW):

Krag rifle stocks did not receive their dated 'acceptance cartouche', until assembled, inspected, and accepted for service.

Looking at estimates, approximately, 21,000 to 26,000 model 1896 rifles were built from January, 1898, to the point in July, 1898, when model 1898 rifle production commenced.
The serial number range for these model 1896 rifles would run from about 83,000 or 88,000 to around 109,000.

War was declared against Spain, on April 21, 1898.

IMHO - With an active war situation, it seems likely that over 20,000 model 1896 rifles would have been accepted for service in 1898 and likely received an 1898 dated stock 'cartouche'.