Vertical Depth Control
The "Check Nut" is the Vertical or Depth Index that allows you to control the depth the tool penetrates into the metal you are working on. It can be a chisel, fuller, stamp, texture pattern stamp touch mark etc.
The Quick Change Tool Holder is easily made from a piece of 1" X 3" bar stock that covers the flat area of the ram. It has a 3/4" stub welded on to it that fits into the ram tool hole and is held there by the set screw. The tool holder goes into the ram and it has a 5/8" deep hole that holds any kind of tool that you can design (chisels, fullers, side sets, cutting hardy, texturing or stamping tools, embossing tools, the list goes on and on) and it has a set screw in it to hold the tool in place. I had some S7 in 5/8 ", so that is why I chose this size. The ram hole is 3/4" and you can put tooling in that also.
Important: An integral tool holder for quick change of tools is much better than one with a welded on stub that goes into the tool hole. If you go with a welded stub make sure you have a good weld all the way around. I would also use a thicker piece of steel than a 1". I'd go with at least 2".