If the metal screw has a rubber 'o' ring it will be the vent screw for sure.. The mast base hole should just be a threaded hole with nothing screwed into it.
This pic shows the V (vent screw) and the M (mast base screw hole) in the old 11'6" Nalu... I'm pretty sure I've got this right.
The board with the oval grey deck grip has the original mast track.. not just a screw hole.. and the grip is stuck over the vent screw so it was permanently sealed.
DJ