1. The markings are definitely "off". I have no idea of the origin of the rifle and won't even speculate without better pictures and information.
2. Eddystone made over 1,376,000 Model 1917's. If it says otherwise, Wikipedia is wrong. (Not for the first or only time; it is a good quick reference, but not one to depend on as the only source.)
3. Obviously, the OP can find many Eddystone Model 1917's in the U.S., but that is the ONLY one he captured in Afghanistan. If it is a fake or copy, that is not bad, it is even better because it is unique, not one of 1.3+ million.
4. It is my understanding that GIs have been allowed to bring back guns that qualify for antique or Curio and Relic status. That rifle is not an antique, but would be a C&R. If I were the OP, though, I would assume the markings are correct for "bringback" purposes. If they are not, then the question may arise as to when it was made, etc., etc. If they are correct, and you get my point, bringing it back might be easier.
Jim