Name that bolt! (pics)

I ran across this thread the other day while searching for something else. It got me curious so I went out and checked and my 273 has 4 solid bolts(unless the hole is completely slugded up)and the plate doesn't appear to have a groove. thoughts?

Both my 67, 273 and 68, 340 have 4 solid bolts. I`ve only seen the bolt with the hole on a couple of occasions.