Can you ID this RCA B/W set?

[kbmuri]

On the 24-inch set, the right-hand (channel) knob is significantly left of the right edge of the screen-bezel frame. On the 21, it's nearly flush with the right edge. I took the liberty of enhancing the knob on your original photo (attached). It's flush like the 21. It's a 21-inch set. Model 21-S-5XX series. Unfortunately, the Sams index lists about 30 sets between 21S500 and 21S510 -- but fortunately there's only 3 sheets covering them all. Sheet 277-10, 282-13, or 294-10. Maybe pulling those sheets will give you another clue as to which one has the white pencil box.