Spring fuller. Made from though  chrome moly steel 4140 for long life.
This is an essential tool for smiths working by themselves as it coordinates the top and bottom tools. It's used to greatly increase the ability to draw out or elongate hot steel. The striking surfaces are made from the same 4140 steel that many of my hammers are made from.

The tool faces are 1/2 cylindrical at the point it contacts your hot work. Suitable for use with a hand hammer, sledgehammer, power hammer or press .