The metal plate you are describing is a weight plate for the menorah. It helps the menorah to be balanced as it is probably a light weight menorah, and chas ve shalom the oil cups should not topple over, and cause an accident.
Some lachters also have the weight plate on the bottom.
Have a Gut Shabbos and a lichtig, wonderful Chanukah with many nissim and an abundance of HAGEFEN for all klal Yisrael!!!