Your description of the rifle's receiver markings exhibits some inconsistencies which raise serious questions about the receiver. An authentic Rock Island receiver ring should be marked "U.S./ROCK ISLAND/ARSENAL/MODEL 1903/serial number". Depending on how the scope mount is positioned, perhaps only the "U.S." and original serial number should be visible. The original serial number is in a much larger font than the other markings. The scope mount should not cover up the original serial number.

The "SF100xx" serial number suggests that the original serial number was ground off and the receiver re-serialized. That might raise legality issues with the BATFE. Your description raises suspicion that the receiver might not be an authentic Rock Island receiver, but perhaps a reproduction M'03-A3 receiver.

Stay safe!

J.B.