You have what appears to be a genuine original 30" M98 Carbine stock, which, if it retains a cartouche it would read "JSA/1898", along with a late 1898 rifle receiver and possibly a genuine carbine barrel of some vintage.

The barrel band is correct for the stock. I cannot tell if the sight (which is of the correct type for that stock/band combo) is for a rifle or carbine.

IMHO, it is doubtful to the point of near-impossibility that SA assembled the arm as shown, unless it was repaired in 1902/03, or later, due to a failed receiver. IF SA repaired the gun, I am nearly positive that they would have replaced the stock with 32" M99 type at the same time.

That said, you have a valuable ($500+) stock, and, possibly a very valuable ($600+) rear sight. Probably the best $25 you ever spent!

You have a number of options at this point.