Looks legit. Nice 911. Has seen a claw mount in its days, along with a sling (as evident by the wear on the front sling ring, forearm sling attachment and buttstock areas). Not quite sure the mag is correct though. Also, not sure what's going on with the shiny spot on the top optic mount area; a bit of filing perhaps?
Tony will swoop in here soon to provide the birth date and blood type of the guy who stamped the extra '1' and reason why...