Those Sonys were excellent TVs. My great-aunt owned one that lasted 20 years; when it finally quit, she bought a new, larger one that lasted until her death in the early '90s. I guess she was sold on Sony.
I can't imagine why the UHF tuner in Jon's Sony TV goes to channel 999.
Those sets were made long before today's digital cable, which does have channels in the hundreds and even thousands (the ones in the 1000s are often HD channels). The UHF tuner in my great-aunt's Sony tuned to channel 99, which was some 17 channels above the top of the UHF TV spectrum at that time ('70s). I never knew or even gave much thought to why the tuner in her set went so high; again, as with Jon's TV, it didn't make much sense that the tuner went so far above channel 83. Perhaps Sony was designing its '80s-vintage TVs for an expanded UHF spectrum that never materialized until two decades later.