After a number of years, I finally tracked down the schematic for this set! I've been importing some service data compilations for Japanese-market TVs, and one of the volumes has the TS-103 in it (along with many of their other early sets, back to their first, the TV3-14T). I suppose I've got to eat three-year-old crow; TV-collector was completely right about its age and I was wrong! The release year is listed as Showa 36 (1961)
Good to finally have some concrete data on it in case it breaks in the future. But I have to say, it has been strikingly reliable aside from some fiddly tube sockets
Full-res picture of the schematic here:
https://imgur.com/a/xbwnyf7