1 won Korean?

Hello, I have this South Korean 1 won 1966 coin, it has a normal first side and a smooth second side as shown in the photo, I would like to know if this is a minting error, because I examined the smooth side and found that it is not due to wear or deliberate action. 

Thank you

Try to weigh it on a precision jeweler's scale. You'll probably find the coin too light because of the missing side!

N#5993 

 As you see on that page there is a border both sides - 

your reverse does not have that [in the lower, darker section] so 

it has been smoothed away, at a slight angle.