After more hours of searching than I care to think about, I'm logging this one as a DNF. The large metal pole in the description is nowhere to be found. There is a large wooden flagpole at the old fort site at N 35° 23.263', W 094° 25.950'. Several conversations with Rangers from the Historic Site leave me convinced that this is the only large pole that has been anywhere near the old fort site for at least 25-30 years. Working from that assumption, I did a thorough visual inspection of the area 355 feet from that flagpole at bearings from 220-230 degrees. No disc was found, but there is a good deal of dirt covering the area. A more thorough inspection with a metal detector might uncover something. I did find something that resembles a railroad spike in the bedrock outcrop in this area, but there was no disk. The railroad spike thing can be found at N 35° 23.218', W 094° 26.006'.
On the assumption that there was a transcription error in the description, I also searched an area 355 feet from the flagpole to the NORTHwest of the flagpole. This area doesn't get anywhere near the bedrock outcrops or near the river. The only thing remotely promising was a green metal post, of the type that is sometimes used to support a Survey Marker sign. Unfortunately, I spoke with the man who placed the post (don't remember his name, sorry) and that post is there simply to hold up the chicken wire around the tree that it sits a foot or so from. He used it simply because he ran out of the orange posts he used on other trees in the area.
There IS a benchmark disk in the area of my SW search.....but it's the wrong marker. US Army Corps of Engineers marker POTEAUNAV is located at N 35° 23.195', W 094° 25.984'. The easiest approach to this marker at this time is from the tourist trail, through some reasonable brush that includes a small patch of poison ivy. At this time, it's not possible to approach this marker from the north along the rock outcropping.
Photos to follow as soon as I can find where I put them.