[Yosemite Sam]

I'm guessing that the pic is of a 100. One of my 70's has a hole in the same location (except on the left side) I didn't see a hole there on the right side, but the hydraulic lift is kind of in the way. I didn't go down and look at the other 70 or the 100 to see if they have it or not.

For what it's worth, often times there will be a hole punched in the "blank" that the part is made from (in this case the frame rail). There is normally a locating pin in the first die used in the stamping process. The blank is positioned in the die with the locating pin through this hole, it insures that the blank is positioned properly in the die before the press is cycled. Many times there will also be a locating pin in several of the dies but almost always in the first die.

Further, this same hole is sometimes used in the assembly process for alignment purposes in the welding fixture, for lifting/transporting the part, or for holding the part while other parts are attached to it as it moves along the assembly line.

It is very uncommon for any of the actual bolt holes in a part to be used during the assembly process for anything other than their intended purpose. For instance, they wouldn't drag the frame down the line using the engine mounting holes or the clutch pedal holes. Oddly enough, the original locating hole is almost never used for anything in the finished product.

That's all I know about that.