1965 220 SE Cabriolet Rear Seatback

I just reinstalled seatbelts in my recently acquired 1965 220SE Cabriolet. Needed to do this so the car would pass inspection. The problem is that I can't seem to get the rear seatback to fit properly again. The sides (with tabs) fit flush against the rear shelf. The middle now has about a 1/4 inch gap between the seatback and the rear shelf. I know it's not right because it was not that way before I removed the seatback and it rattles quite a bit. Is there a trick to getting the seatback to fit back snugly? Any help would be greatly appreciated.

Thanks very much...Matt