That would be the barrel selector... AGAIN, the button the side is the "safety"
DM
I assumed that was a barrel selector it threw me off the first time I looked at that gun . BTW..that has to be the worst safety design I have seen but if it works for you. Furhtermore in order to disprove my point you bring up a photo of a gun made early in the previous century. Does any modern o/u or SxS have a side safety and the answer is clearly no.