Oh, a real newby!  Those are "bench pickles" you are talking about.
Most of them are vaguely pickle-shaped and have a slight bend like
a pickle, too.  There were wood and metal versions.  Some benches had
installed, on real hinges, a board about two inches wide and as thick
as the bench side, that flipped up from inside the bench to rest on
the side wall of the bench.

There were lots of benches also that had a fixed slanted top on them.

D.L. Bullock    St. Louis
http://www.pianoworld.us/