The HD-801 Fitting Guide is made with file-resistant buttons having a radius of .242" for use when installing a Harrison Design grip safety. Also included is a smaller diameter spacer that is installed between the frame tangs to keep you from squeezing the tangs together if you tighten the screw too much. Be aware that while the guide buttons are "file-resistant", 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 with the smaller diameter spacer in between the frame tangs. If the spacer is too wide for your frame, check to see if the beavetail that you are installing will fit. If the beavertail fits and the spacer doesn't, file the side of the spacer down until it just fits in. If neither one fit into the frame tang gap, then widen the frame tang gap until the beavertail fits. Then check the spacer and if it's still too wide, file it to fit.
Assemble the HD-801 on the frame, push the buttons to the rear as you tighten the screw, then use a sharp scribe or needle 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 Dykem. 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 by filing around the tang's radii, rather than across them.
