[Ron Ramirez]

Well, Chuck has already explained it very well; not only here, but on his site also. Here's the link:

http://www.philcorepairbench.com/tips/svctip35.htm

For now, I can add that there was sometimes no rhyme or reason to a change in a Code number, or lack thereof.

Model 60 is a prime example.

As explained in detail on my site here:

http://www.philcoradio.com/tech/60evol.htm

For the entire run of Model 60, it remained Code 121 even though it changed a LOT during its nearly three year production run.

And then there is Model 89. Code 121 = AM only. That is clear enough. Code 123 = AM and shortwave (1.5 to 3.2 mc). What isn't so clear is that Model 89 became Code 123 when the "police" band was added (March 1933), 1-1/2 years BEFORE the Type 36 det-osc tube was changed to Type 77 (September 1934).

(edited to clarify timeline)