'69 340 engine

It is for the camshaft thrust plate. The one with the hole goes into the top hole on the passenger (even bank) side. This allows oil from the lifter galley to come through and oil your timing chain. There should be three more just like it without a hole for the other holes on the thrust plate.

They have used a few variations for oiling the timing chain. One was the bolt with the hole in it like you have. Some engines had a bracket that bolted in three of the holes for the thrust plate and a tab that went into the passenger (even bank) lifter galley and you did not use a bolt in that hole location. The third way was to just use three bolts on the thrust plate and leave the bolt hole on the passenger top open for oil to come through.