does the hole your refering to have a 1/2-3/4 inch bung, with a thick rubber cap.... a bung the same size as the L-shaped oil return lookin rig on the oil pan???
if thats what you guys are talking about...
those are bungs left over from the rally car days when mazda ran a sort of PCV system like a jetta uses... with a pump to evacuate crankcase gasses.. i think it was experimental, for WRC...
ive got a block in my garage ill try to get a photo of the back side of it.. see if we can't solve this mystery...