The fact that there's a slot there implies a press-fit knob. However, none of those type of knobs seem to fit the "quality" of the pot. Further, unless the knob hole is perfectly matched to the shaft diameter, a slotted shaft causes even more wobble when the set-screw is tightened. With the length of shaft that several of us have established as the Head-Fi DIY standard (RK27, 50K, 25mm shaft), most of the time the slotted portion is cut off, anyway.
Just an FYI, but AFAIK - all the 50K ALPS RK27's are a special manufacturing order (iow, high volume). This is further complicated by the Japanese tsunami. The ALPS factory was hit.