The tuner modules were purchased outside and there probably was a production change (or maybe they used whichever they could get?). Module with 17C9 is interchangeable with the one with 12DT8 - same input, output, same 150 mA heater current.
As it happens, I have TWO of the Silvertone / Arvin STEREO radios with a similar tube lineup. Schematic is here:
http://www.audiophool.cjb.net/Misc/Ag4019-2.gif
http://www.audiophool.cjb.net/Misc/Ag4019-2b.gif
Mine have the tuner with 12DT8 - schematic shows 17C9. The oddball output tube (50HK6) was used due its tapped heater, which gave a place to connect a pilot light.