You're welcome for the assist. Numerous documents, official and unofficial, confirm that USAT Willard A. Holbrook carried not only the entire 147th Field Artillery Regiment, but the First Battalion, 148th Field Artillery as well. The latter battalion had been Idaho National Guard.
If you've not read it, I recommend Richard Cropp's "The Coyotes: A History of the South Dakota National Guard", published with the cooperation of the South Dakota Board of Military Affairs and the National Guard Officers Association. Bill Heath wrote a similar history of the 148th Field Artillery, in its second edition assisted by Gayle Alvarez, Idaho Military Historical Society. I think Heath's history focuses on the First Battalion, rather than on the entire field artillery regiment.