[Mondial]

There is nothing really wrong with the shadowmeter box itself. The problem is the position of the lamp filament in relation to the hole in the back and the vane.

The original bulbs had a filament location closer to the lamp base which was properly positioned with regard to the hole. Modern replacement bulbs are different and the filament is rarely in the right place. You may have to go through quite a few bulbs until you find one which will center the light beam over the hole. Alternately you may have to bend the lamp bracket for best alignment to center the shadow.