[Radioroslyn]

The 16B code 121 uses a tone control that adds .003mfd at each position to the right it clicks. The first position open with no connections to the caps. There is a pretty drastic difference between the 1st and 2nd positions. 1st is very bright and 2nd is kinda muffled. It's a bit of a pain to take the tone switch apart to change the value of the caps so by adding a resistor in series with the 1st .003 cap you can lessen the effect of the 2nd position. I ended up using a 120K resistor. Makes a smoother transition between the 1st and 2nd positions doesn't sound anywhere as near as muffled.

[Arran]

Another option is to substitute a pot for the switch, along with adding the appropriate circuitry. I think Philco used these switches because they could make them in house, and they were therefore, cheaper, or because it was a way to avoid paying royalties to RCA, or some other outside firm, with the same result.
Regards
Arran