It is a tabletop. If no one was sitting there, the top of the seatback would fold forward, and the people in the seats on either side could have a table between them.
Eastern did this also when they first installed the widebody look on their 727s, except the table was built into the lower half of the front of the seatback. You couldn't see it because it was covered with seat fabric. You had to open it to get the table out. These disappeared when Eastern redid their interiors. I guess that since most people didn't know they were there, they were seldom used.
Fly Eastern's Golden Falcon DC-7B