You did really well to find yours for $300, congrats!

My 39a circa 1957 is drilled and tapped on the left side of the receiver, two short screws filled the holes.
I put a Lyman 66MC site on mine, ~$80 from a local gun shop.
You're gonna have some fun with the new Marlin!

"Camping places fix themselves in your mind as if you had spent long periods of your life in them.
You will remember a curve of your wagon track in the grass of the plain like the features of a friend."
Isak Dinesen