Like all of Yamaha's early audio units, this was a statement.
The TC-800GL received many honors, museum exposure and praise : it was designed by Mario Bellini and not by Yamaha's usual audio designer (GK Design).
The original TC-800 sported a High Filter and a Microphone on/off switch ; that version was also produced in white but apparently not beyond a few pre-production samples for early (japanese) catalogs.
The TC-800GL replaced the 800's functions with a Dolby IC and a Mic/Line mixing and the addition of a limiter switch ; it could operate with a DC 12V power source or with nine 3V batteries.
A third version, TC-800D, was an updated TC-800, with an added DIN connector, auto CrO2 tape compatibility and Dolby IC.
The TC-800D eventually replaced both original versions before vanishing itself quite rapidly - design slants were getting rather different in 1978.
In the same vein, one should recall the Leak 2002 tape deck, the Lecson units or the Nakamichi 600 & 610.
But all in all, slanty slant didn't fare too well : difficult to mix with other components and more expensive to build.
Yamaha didn't pursue the slant anyway but nevertheless did manage to insinuate very slanted surfaces in many of its late 1980s amplifiers, tuners and CD players.
The TC-800 was built for Yamaha by "CPLK CO. Ltd.", aka Copal Koki, which also built early Luxman cassette decks...
More images of the 800GL (including the Invisbilia white version of the TC-800) and a very pensive Bellini here.