Welcome to the KCA forum.  What is the serial number on the Model 1898 receiver?  Spgfield, as you state, those early parts cannot be correct on a rifle built in 1903.  1901 and 1902 rear sights and hand-guards are not difficult to find over time, but, expect to invest another $140.  However, you could recover most of that expense by selling off the earlier parts.  Maybe you can leverage the 'wrong' parts to get a better price.