Actually, that plug is to cap off the hole where the passage was machined for the rocker shaft oil supply. If you look in the hole in the pix, you can see the head bolt that goes through it. I would say that if there is an oil supply problem that a po was trying to fix, it's either the that thre's a blockage in the supply ports in the block itself, or the line that runs in the lifter area is crimped/blocked.
