Earthwood 6 string acoustic mid ‘70s bolt on neck adjustment:

Bought in California in 1976 or 77. I have used it intermittently over the years and am looking to adjust the neck angle. I know there are two extra small holes which pass from the back of the guitar all the way to the backside of the neck, but I have no idea what kind of adjustment screw is at the bottom of this 5 1/4 inch deep hole, nor do I know anything about the geometry of the actual joint system. Anybody out there have any experience with these unusual but, to me precious, guitars?