That is an example of form over function (I am being charitable here).
Best replaced entirely, with something that will let you get a good grip on both ends.
You might find a bog-standard seatpost-bolt nut of the recessed variety will fit in the frame socket; if so file off the tang and cut it carefully to length, such that it bears on the internal flange of the frame-socket, not on the rim (leave a little gap there to allow for wear).