That screams 'Metric' thread to me. 5/16" is, as near as makes no difference, 8mm. 8mm screws are commonly (unless they are fine thread) 1.25mm pitch. 1.25mm pitch is roughly 20tpi. Being as the phillipines are part of the rest of the world and the rest of the world uses metric fasteners, I would lay odds on this being the case.
Checking this against a metric thread gauge, as I have suggested, would solve this mystery.
dld