For engines that were made from 1974 onward [rear distributor] the rocker oiling comes upward via a drilling in the block, to another angled drilling that takes it to one of the head bolt holes.
This particular head bolt is longer as the threaded portion of the block is lower down, or deeper into the block. This means that the hole can have some of the thread drilled out completely & the bolt still has enough thread engagement.

Now we have a space around the actual bolt shaft that effectively allows oil to pass up alongside the bolt into the head. It doesn't come out the top because the washer & the bolt head form enough of a seal to prevent serious loss.

There is another drilling in the head that intercepted this head bolt hole & takes the oil to the base of the centre rocker pedestal where it continues to travel upward & into the hollow rocker shaft,&, ... well, you know the rest.

As I said before, this particular head bolt is longer than the others as it needs to reach the threads deeper down in the block & I think that it is 'waisted' down to the minor diameter of the threads to allow maximum clearance for oil transfer in a standard diameter head bolt hole.

The A10 & 1200 engines, up to 1973, used a separate drilling in the block & head to achieve the same result, but it was deemed preferable to change this when the engine was re-designed for the '74 model