This punch is specifically designed to create nakago-ana (tang opening) in iron or copper tsuba, saving time with a cold chisel and files. The concept is to forge something shaped similar to a tang but with an exaggerated taper for strength.
The tool could be hardened but will likely lose its heat treatment during the drifting stage so best to keep the neck sturdy and short enough to hold up either way.
Working at very high heat will help prevent splitting when punching wrought iron. The tsuba in this video is medium carbon steel.
As time allows, the plan is to forge a bottom die (rather than use the hardy hole) to reduce the amount of distortion at the edges of the nakago-ana and speed up the drifting process, reducing the required number of heats to drift.