Y'all are right-- It's an inkwell. I have read a vintage ad for this model cabinet (somewhere online!) and it was sold as a dual-purpose sewing cabinet and writing desk. There is also a paper tray installed on the right side, behind the door and above the two big drawers. This tray does not pull out like the drawers, but has a cutout where you can reach in and pull out a sheet of writing paper!
There are variations of this desk. I've seen it in at least three colors, and the one I own doesn't have the inkwell, but rather a six-pin rack for thread spools in the inkwell location. These cabinets came with only the 201 or the 15 installed in them. They are wired and have their own foot controller built into the cabinet.
This is the "Art Deco" cabinet...