It looks like one of the two screw holes that secures the road tubes oil deflector plate. If so that means the plate wasn't installed by whomever worked or rebuilt the bottom end.
The purpose of the plate is to deflect the oil being splashed around via the rotating mass in the engine from directly entering and then exiting out the road draft tube.
It's an easy to overlook minor part on a rebuild, unfortunately I know....
The down draft tube secures at a different place, lower and towards the distributor
see pic. It is plugged with the brass colored bolt in the pic.
Michael...
