I just finished setting up an O.S. 1.20 pumped with a Slimline.

The O.S. comes with a muffler which is predrilled ( ostensibly for the non-pumped version of the engine ) at the point where the muffler begins to widen out just past the threads.

This was close enough to the threads that I elected to try it this way.

It's worked just great, especially since the stock muffler also has a hard baffle like structure in the metal that helps keep the oil in the muffler a bit longer.

So I did not have to drill anything, I merely removed the existing screw.