The small internal water passage is to cool the divider between the exhaust valves.
The screw head is the drilling of the oil passage feeding the head. As you can see the DOHC heads have 4 longitudinal oil passages, much more complex.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.