I'm not sure why it is square, but that hole is actually how the draft hole was drilled - at some point inside the shank, the two holes merge. Think about the geometry - drill bits are straight...
Pipnet, oyster knife, senior reamer, and a triangular machinists scraper is what I use along with a set of chip carving chisels to clean up an "estate " meer - followed by sandpaper on a sharpie.
I run a 5/32" aircraft (that simply means long) bit through the shank on all my meers and a 1/8" through the 2-piece mortise / tenon adaptor - that gives me the perfect draw...
I use a set of chip-carving chisels to persuade the carbon away from the meer... Just remember, it's not a race - too little is better than too much...