Here you go modular maniacs.
Most of you guessed its application, but not location and for good reason.
This is the first time ive seen an oil plug look like this from a modular head. Its use to plug the oil gallery where the timing tentioner actuator resides. You see, the heads are interchangeable left or right side and is completely machined to be used for either side. All thats needed is to insert the all plugs in the correct location. This plug in particular caps off the tentioner oil supply passage on the rear of the head.
I find it odd the plug to look this way, cause its the first ive seen, but given most of the mod heads i have laying around are pre-02 and these are '09.
After inspecting why the plug looks like a restrictor, im still clueless.