Date Nail Question

I have a bunch of SG ties that I am decorateing the place with and many of them have date nails in them. The oldest I have noticed is '33. Most of the dates are in the mid '30's. I am pretty impressed with the age of these ties and every time I see a black spot I think about the hot ember that fell there no telling how many years ago.
It has even crossed my mind that the men that laid these ties are almost certainly dead now and many of these ties are still very solid. It amazes me how well they have stood up to the weather in West Texas.
My question is, what does a nail with an "X" mean? The one I am thinking about is adjacent to a '33 nail.
Thanx,
~Hoss