Did you get permission to post the serial number from the owner? Many owners of NFA items do not like you doing that. Looks like it was the 15th rifle of roughly 80,000 fo the Colt M16 Model 640 (Export model of the XM177E1. You shouldn't expect perfect from a 40 yr old machine gun, if it was refinished it would have been years ago. What you posted should sell at a premium to most (rarer model and better than average condition).