No one posted an answer yet to my inquiry on the OAN discussion forum, but from the Submission requirements, "board" is singular:
2. Presentation Board @ 24"x36" (REQUIRED)
The presentation board should clearly display the entrant's approach. The classroom planning and design, approach to sustainability and material use should be included.
Additionally, the organizers will use this image as the primary board to display in exhibitions, publications and in community meeting in the field. This board MUST be the first image to appear in the slideshow for jurying purposes.
The file should be named/saved as: [entrant ID number]_01_mainboard (i.e. XXX_01_mainboard)