From the record
“Inscription "B" is a survey mark that was left at the confluence of the Colorado and Green rivers in Utah in 1889 by the Robert Brewster Stanton party as they surveyed a railway route.”
Wikipedia article D.C.C. & P. Inscription "B", CC BY-SA 4.0.
One of 18 National Register listings published here for Wayne County · listed in 1988 · category: Site · built in 1889.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service