Fabien,Ed,Frogvalley,
This really isn't a mystery hole - it is an area that long stock that is being forged can be passed though. You'll notice the dies are perpendicular to the frame, this design was changed as years went by - as in the little giants - so the dies were turned at an angle so that wagon wheels could be welded with the power hammer, and the frame would not get in the way. I have a 50lb Little Giant that was made in 1900, and it has this same hole in the frame (which is level with the bottom die), and my 1912 model 50 has the dies on an angle. This information was read in the book "Pounding out the profits" If I recall correctly. I'm sure others will agree or have a better description of what it was used for. -JK