The hammer fitting block is for the final mating of the hammer to leveled strings. 
 
The tool has four sides:
One 1/8 inch for removing felt from the left or right side of the hammer.
An 1/8 inch strip for felt removal of the center string. 
The third removes felt from either two string strike points.
The last is as wide as the hammer.
 
The sandpaper is 120 grit. The tool is plastic which allows the contact sand paper to adhere to the tool much better.