The HD-801 Fitting Guide is made with file-hard buttons having a radius of .242" for use when installing a Harrison Design grip safety. Be aware that while the guide buttons are "file-hard", you can still damage the surfaces if you grind against them, or otherwise abuse them. Also be aware that the screw is very hard to source, so good practice is to keep the Fitting Guide assembled when not in use.
Assemble the fitting guide through the thumb safety pivot shaft hole in your frame, and use a sharp scribe to mark the radius on to each side of the frame tang. Remove the guide and use a belt sander to rapidly remove stock until you get close to the scribed line.
Once you have removed the excess frame tang stock down close to the scribe mark, reinstall the fitting guide and color the buttons with a Sharpie or Dukem. Do not overtighten the screw, as you can pull the tangs toward each other if you do. Hold the frame in your vise and use a 6"-8" mill smooth file work across the frame tangs to take down the rough-cut tangs until you start touching the guide buttons.
Your tangs will now be within approximately .002" of finished radius on the tangs. From here, you will use the traditional "spot and file" technique to do the final fitting between the beavertail and the frame tangs.
