The mint didn't use that metal composition in 1979, so that is very unlikely. The only chance it has of being genuine and unplated is to have the cu nickel layers missing off of both sides which would be very unlikely but still possible.
Weight doesn't really prove anything in this case and the matte surfaces look more like a plated coin that a struck coin. Take it to a local shop and see what they say, there's no way for me to say one way or the other if it's real or not based on an image.
Impossible since that alloy didn't come to the US Mint until 20 years after the SBA was minted. You have a coin that appears to have been messed with.
There are SBA's struck on SAC dollar planchets, but they are all 1999 SBA's. You're right though that it is impossible for it to have happened with a 1979. Since the copper color is on both sides of the SBA and the weight seems to be probably within tolerance specs, I think we can eliminate missing clad layers. That would leave environment discoloration, plating, or a sintered planchet.
