One of the many mid 70's Sony cassette recorders which, like most others, had a short availability depending on the version or market.
The 204SD seems like a mix of the TC-4350SD (1974) and TC-2350SD (1975), the result of which was probably remixed to make the later TC-K4 or TC-K3 (1976).
The main novelty, in 1974, was the "visible front loading" and the abandon of the flatbed / console look.
The features are classic if somewhat plethoric for such a low/mid-end item : separate bias & eq switches, built-in limiter, Dolby and/or 19Khz MPX filters, F&F heads (two), lockable pause key for cueing, stereo mic/line mixing, separate line out and front binaural jack input on top of the regular L/R MIC jacks.
Plus three LEDs for peak level, record indication and Dolby function - the TC-177SD had five LEDs... but cost three times as much !
And "handsome matching components available".