That I'm aware of, the STR-7800SD was only available (sort of...) in the North American markets and not at all in Europe where the more common (but already worthwhile) STR-6800SD was sold as lineup topper.

Sony's top-end units were (and still are) scarcely distributed (if at all) so, like many others, unfortunately, the 7800SD is one monster receiver almost no one has seen. I'm not sure (reaad almost sure) the STR-7800SD to not have been available in Japan - that country never was really receiver-oriented...

Pity - because the 7800SD sported such ultra-high-end componentry as a (huge) toroidal transformer and LEC transistors - near science-fiction in 1975 !
Plus OCL outputs and DC circuitry, a PLL/MOS-FET 4-gang tuner section, three speaker pair outputs, an adaptor loop, Dolby FM compatibility, lots of power and supersized (but rigorous) looks to match the inside.