This is an old question, but I do have an answer. This "Art Deco" cabinet was sold to house a Singer machine (e.g. 201), and was advertised as a letter-writing desk/sewing machine cabinet. It was intended to be a dual-function cabinet, back in the days when women actually wrote letters. The piece you wondered about is an ink well! You'll notice on left drawer edge, there is a place to store your pens. Some models also had a slot in one deep drawer for the writing paper.