Does anyone know what this is?

[TBKS1]

So I was looking around Google Street View earlier today, and I saw something that looked like a very small brown sign with a special number on it. A few of the roads were labeled as "NF" or "NFD."

Does anyone know what this could be?

Some examples:

https://www.google.com/maps/@34.5408852,-93.3503102,3a,16.9y,274.08h,77.3t/data=!3m6!1e1!3m4!1sBo2cOhSPI7_MhhoUw1NEAQ!2e0!7i3328!8i1664

https://www.google.com/maps/@34.5243048,-93.4148445,3a,15y,285.28h,78.26t/data=!3m6!1e1!3m4!1sKIZ6RIq1sWy-e9VsvLnvjA!2e0!7i3328!8i1664

https://www.google.com/maps/@34.3994938,-93.8310174,3a,15.4y,336.87h,87.17t/data=!3m6!1e1!3m4!1s6M7bzxS5iAZYPURvUN0V6Q!2e0!7i3328!8i1664

[dfilpus]

National Forest Road and National Forest Development Road. The brown signs are giveaway.

[TBKS1]

National Forest Road and National Forest Development Road. The brown signs are giveaway.

I kind of thought it might be that, but the route shields don't look like NF highway shields, so I just wanted to make sure.

Thanks!