The gunshop owner is misinformed. I agree with Calif-Steve. A few case-harden receivers did "leak" through, but you're pretty safe. The barrel is probably original, but the rifle appears to have been refinished. Still, a nice example of a M1903 that has been around the block, but looks very presentable. The Hatcher Hole was used by the Army, as well, and I don't see any other USMC characteristics. Can I ask how much you paid?