[bandersen]

I suspect you are correct about it having been serviced. There are three 12SN7 tubes and they do not match each other.

I replaced the 12AT7 mixer oscillator and placed a shield on the 6AG5. It seems to be stable now. Will periodically run it a few hours over the next few days to make sure.

Coincidentally, I picked up a cosmetically minty 9T1 of my own in July, 2019. I have not pulled the chassis yet but only one tube appears to be Motorola branded.