It's an alternate method of getting oil to the rocker arms if the block passages get plugged up. A 3/16" line is run around the back of the engine from the oil pressure port and tapped into that center port in the head. If you're getting good oil flow to the rockers from the internal passages, leave it plugged.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!