This intriguing oddity appeared on ebay recently:
I am wondering if in fact this piece is complete as it is, and was used to heat the metal plate. Possibly for sad irons to sit on? But one would think irons of this sort had long since gone out of use.
Per the auction information: dimensions are 12 1/2″ Long x 9 1/4″ Wide x 4 3/4″ Tall