Jim's right. I have the same ""M" prefix rifle that has the 4 drilled & tapped holes on the barrel. Somewhere I also have a copy of an old factory Marlin letter explaining why they did it. I beleive a common Weaver N2 mount bolts right up to these holes. A painless way to check to see if they're factory is to remove a few of the plug screws and check that the inside of the threaded holes are blued.
GLWS..
