The fret board extension is extremely fragile. Since we work with removed necks, it is easier to install all frets while the neck is off.
We developed this fretting anvil to reduce breakage. As an old house carpenter, I learned many years ago that you cannot pound a nail into a 2x4 that is not solidly backed up. If the 2x4 is wobbly, the nail refuses to go in.
To solve the problem of fret hammering into that floppy fret board extension, we made an anvil.
By the way, the anvil is sitting on a derelict Gibson MK-35 top.
Yes, that is a chunk of railroad track. We surfaced (flattened) one end. When placed under the fret slot, hammering frets is now stress free.
You can see the anvil just peeking out beneath the fret board extension. Works great.