EDIT: I was replying to the picture of the bare block. You posted the second pic while I was typing.

It is either the red or yellow circle. Double check when you are under the car as the picture angle can be deceiving.

If it is the yellow circle, try tightening that plug with the appropriate sized allen wrench to see if the plug loosened aver the last 30 years.

If it is the bolt securing the water pipe in the red circle, also try snugging it down. If it is the red circle, the pipe could have developed a leak behind and above, where the picture looks like it originated from the bolt.