The brass buttons where intended to be used with a track trip. The trip was not produced for some reason. The buttons only appear on 1946 locos and dropped in 1947. I have a 1947 Hudson with a 1946 chassis. That means it has the holes for the buttons but no buttons. Also it has the cut out behind the from drivers.
also gone after 1946 was the use of round head rivets on the linkages.
jake