As stated above the nut is probably there for another application. Farm equipment or the like. I am guess that the “chrome” valve cover was not used on many pieces of farm equipment.
It gives the engine more nuts for getting through the slop. :lol:
I'm just teasing you. I read somewhere that the nut was there to hold a bracket for optional accessories. It was probably when the engine was used in the agricultural field. Not your's specifically just that run of engines.