The API 5B1 is a single-slot blank panel, used in the API Lunchbox, 500V Rack, and expansion slots for the 1608 console.
Nobody likes an open space in their racks, which is where the API 5B1 comes in. Until you can fill up your API Lunchbox with goodies, the 5B1 covers those nasty-looking gaping holes. Matte black finish.
Order your 5B1 now and fill those blank spaces until you can replace them with some 500-series preamps, EQs, and compressors.