Doug,
Great score! It's great when you can find vintage sets like that as cheaply as you were able to get yours. One pays $3 or more these days just for small-signal tubes like 12AU7s, et al., and the larger tubes such as 6DQ6s and the like can go for a lot more. Just goes to show how rare vacuum tubes are these days. But to get an entire TV for the same price . . . The only way to get these oldies cheaper is to grab them from the curb on trash day, or if you get them free from someone who wants to get them out of their house, etc. ASAP.
The front panel of your set looks very familiar. I used to know someone in my hometown who had a TV like that when I was a kid in the '60s and early seventies; yes, it screams RCA, even though it has the Silvertone name above the channel selector. Can't remember the chassis number for the life of me, though.
The design was popular, however, right around the time period you mentioned in your post (mid to late 1960s). I believe very strongly it is a rebranded RCA because of the control layout. The only thing missing is an illuminated channel selector window above the channel knob itself (most RCA consoles and some of the company's better portables/table models of that era had this type of channel indicator, rather than just a knob). Other than that, your set seems to me to be RCA all the way.
BTW, I saw the pictures you posted of the Zenith plant in Chicago; nice to know Zenith still has a presence there. Even though Zenith makes most of its consumer electronics products overseas these days (I used to have a Zenith 8-track/cassette/phono/AM-FM stereo system that was actually made in Korea; my system was made in the early '70s, so perhaps Zenith already had moved its audio manufacturing facilities there by that time and had kept its television plant in Franklin Park, IL until the late '80s or nineties).
One question, though. If Zenith now has its television and consumer electronics manufacturing plants in Korea and elsewhere in the Orient, what is in all those buildings in Chicago? I'd say it was office space, but what would one company do with it all? I find it difficult to believe Zenith would need that much space for its corporate headquarters. I won't even try to guess how many hundreds or thousands (!) of square feet all those buildings add up to.
Kind regards,