The part installed at the factory looks like a plain old nail, about six penny, trimmed to length. Plain as in not galvanized.
It's not a big deal to install, but the need to remove the sears to pull a fox stock makes it tedious. I would leave the sears out while you fit the length because it may take a few attempts to get it right. Maybe someone will measure one that is out of their gun to get you started. But like everything with these guns, you'll probably have to tweak the fit.
I have been inside the workings of a Fox double several times. Recently, I learned that on a 12 gauge Fox that needed an auto safety rod, that I did not have to remove the sears to add an auto-safety rod. I just removed the trigger guard, lower tang screw and top lever screw. Then pulled the stock rearward and tipped upward exposing the guide hole in the stock, on the left side, for the rod. I "slowly" shortened the rod with a belt grinder until I reached the correct working length of 1 3/4" for my particular shotgun. You will know that the rod is too long when the top lever will not move to it's locked right position. Reinstalled the stock and it works fine.
Rapidfire
