Thanks for all the help folks. I was able to find a schematic for the 10S669 and it has written on it "Also Model 10S668". I'm really surprised that NA doesn't have either one of these unless they're listed as something else. I'm noticing differences between my radio and the schematic that I have. On the six tone select switches for instance the color coding of the wires doesn't match. Also several of the resistors on the schematic are listed as 68M Ohm, 47M Ohm etc. The resistors actually read 68K and 47K. Did Zenith use the letter "M" to denote a thousand? Also does anyone know where I can get a schematic specifically for a 10S668 with a 10B1 chassis?

There is a schematic for a Zenith 10S669 (chassis 10B1)on this site - there is, however, a typo and it is listed as a 10S699. I think this is your radio - the chassis number is likely the important point.

Yes, on some earlier schematics "M" (the Roman numeral for '1000') represents the current "K" (kilo) ohms rather than the current "mega" (Greek for 'large'). There's always been some monkey throwing a wrench into the nomenclature.

Not sure about the color code to the tone switch - if it works, don't worry about it. Those wires have a bad tendency to fade into completely different shades over time.

:There is a schematic for a Zenith 10S669 (chassis 10B1)on this site - there is, however, a typo and it is listed as a 10S699. I think this is your radio - the chassis number is likely the important point.
:
:Yes, on some earlier schematics "M" (the Roman numeral for '1000') represents the current "K" (kilo) ohms rather than the current "mega" (Greek for 'large'). There's always been some monkey throwing a wrench into the nomenclature.
:
:Not sure about the color code to the tone switch - if it works, don't worry about it. Those wires have a bad tendency to fade into completely different shades over time.
:
:Very helpful. Thank you!