If my memory is correct the 336 had 2 small holes drilled and tapped on the side of the receiver for mounting peep sights ---I think a Lyman 66 also fits . Yes they were factory drilled and tapped
Yes---Some were not drilled on the side. What I have found is that on some of my own Marlins where I used a Williams sight that was a top receiver mount it tends to raise the rear height (this is at it's lowest position) which requires you to install a higher front sight in order to get a 50 yrd point of aim zero with the rear sight all the way down. On the side mount where I used a Lyman I was able to get away with not having to install a higher front sight blade.At some point the holes on the side went away and they started making sights that used the scope mount screws. That rifle is drilled and tapped for a scope. I cannot imagine someone having it drilled and tapped when all they would have to do is buy the right sight. That being said it is possible that it was not drilled and tapped there from the factory but it is not likely. Either way I would not sweat it if it was done professionally.