I would think that a toplever spring in that particular location would interfere with the safety pushrod which is typically found directly under the topstrap. Also, if it was a peg hole for the toplever spring, one leg of the spring would bear upon the underside of the toplever, and the other leg would also have to bear against something in order to create spring tension to return the toplever to center. What would there be in this location to bear against besides wood? While I certainly haven't seen every shotgun action design in the world, I would be very surprised if this hole had anything to do with a toplever spring. I also agree that it is not an oil hole.
The spring would bare against the lever boss or cam one one side and locate in a slot in the back of the action on the other limb or as on a Purdey or Greener the spring has a hole at the end of the limb and is held in place with a screw into the strap..
