Nikon had several lines, likely from different OEMs. Looking specifically at the Buckmasters, the scope body does look identical. But it's the fact that Sig is using the old Nikon line name that made me even compare to start with.
I had the 6-18 Buckmasters with the target turrets, from when it was their second best line after the Monarch. Some years later the Buckmasters was suddenly a $99 low end scope.