JDM Master Cyl question

[dub40sx]

Picked up a jdm master cyl and noticed a small port between the front and rear fittings. Im not sure exactly what its for. Doesn't have threads, looks like some sort of plug just pushes in.

Any input?

THANKS.

Small black piece looks like a plug I guess?
http://i2.frsimg.com/images/detailed_images/Nissan_46010_45P20_06.jpg