Sony STR-6120
(1967...1970)

I believe the STR-6200F to be the "F" version of the original 6120, making the transition from "normal" to "Fet" and "smooth knobs" to "knurled knobs" along the way. The latter difference was export-only, as japanese units retained the original (and much better looking) smooth knobs, à la Luxman.

Besides FETs, marginally better specs and the replacement of the "tape head" input with a 2nd auxiliary input, the STR-6200F seemingly just added a deemphasis switch on a slightly modified back-plate.

Both models were tagged at 700$ - exclusive stuff which is quite rare nowadays, just like Sony's later STR-7800SD or STR-GX10ES...