That external line was added if the oil gallery from the rear cam bearing area got clogged. That was a common problem on the full pressure 235's which resulted in the rocker arms losing lubrication. The other end of the line got plumbed into the driver's side main oil gallery that supplied the oil pressure gauge and the oil filter. The original intent was to provide two oil passages in the head that intersected at a 90 degree angle to lube the rockers when the under the pushrod cover line was eliminated in the late 1950's. The end of the sideways hole was plugged with a 1/8" NPT plug.
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!