I wouldn't dismiss it just yet, the AHA stamp has shown up before. Not sure who's it was and years ago someone published that it was a USMC stamp, but that is totaly out of character for the USMC and there is no supporting documentation, I would be shocked if it actually was. Brophy mentioned the stamp and simply listed it as unknown.
John Beardicon would be the best bet on this one, see what he has to offer. My opinion of the rifle, I agree with Chuck there's no sign at all that it's been through a USMC shop. Also, the stock is a Springfield (the "S" you mentioned in the cutoff recess).