https://www.dennis-carpenter.com/tractors/engine/gaskets/eaf-6863-c-rocker-cover-vent-gasket

I ordered this and I was wondering where the square hole goes, top? bottom? My 640 doesn't have that gasket.

I misunderstood your question. The part suffix "C" designates there were two revisions prior to so the square hole may have been added later.

The square hole engages a tang punched out of the valve cover. It serves as a drain back for oil that might get blown out the cover and into the vent. I don't have my NAA handy to check but if yours doesn't have the tang, the new gasket should still work.

What tractor are you looking for? Note that part is used from 53 - 64 on an overhead valve engine so if you are looking for an N tractor, the part was not used.

From his question: "My 640 doesn't have that gasket."

Yes, it's a 640, 1957 vintage.

Terry, it looks like original ones from the factory did not have the hole in them:

Not sure why the modern one does???

Well, I dug into the tractor more yesterday and found what the hole was for. There is a square pin (or something similar to a pin) underneath the air vent hole in the valve cover. It aligns perfectly with the square hole in the gasket. So the hole goes to the bottom.