My brother installed, sanded and finished this floor about 15 years ago and faced nailed these planks as you can see in the photo. These planks are 2" thick and some are full length across the rooms and they are fastened to screeds.
We sanded and finished again after this floor had 3' of saltwater during Katrina. The owner and many of the historical societies around here do not want us to fill the nail holes.
I like to fill these nail holes if they will let us. And yes, they still show after filling but they want them to show.
The white lines in the gaps between the planks are not there, just glare from the lighting.