28 Holes in the frame

At the back of the 1928 Hudson sedan, there are two sets of holes in the front-to-back main frame members near where the cross piece is riveted on that goes to the shackles. The set of two holes nearest the tail of the frame are at an angle, or one up and the next down. Then there are some rivets, then the second pair. What things go into these holes? I don't see how it could be the bumper with the rivet heads there.....