You should compare your SN to the one's on IPSND to see if it's a sample or early run game:
http://www.ipsnd.net/View.aspx?id=823
You could also try to contact other owners from the PinballOwners site, or here on Spinside.
If you find out it's different from the factory, you could let Jay Stafford at IPDB and the guy at IPSND also, who's tracks differences in production machine on that site.
Interesting mystery!! Thanks for sharing it...