Later export-only version of the ST-8600 in Technics' then current all-metal brown-ish black fashion.
With one feature added : Servo Tuning.
This ST function is solidary of the 3-step muting switch : when muting is off, Servo Tuning is off, when muting is on, Servo Tuning is on Auto, when muting is on "deep", Servo Tuning is on... auto.
I have no details about Servo Tuning (maybe an early version of Active Servo Loop ?) but it may have been added as an upper feature so as to compete a bit more with the finally not exported ST-9700 monster.
However, the latter was released in september 1974, more than a year before the november 1975 ST-8600... This would make the ST-9600 a two-year late lower-end export version of a lineup only partly exported, by way of an earlier in-between. That's much too Luxman to be true :)
The original ST-8600 had a "Technics by Panasonic" tag, the ST-9600 only had Technics and the little "National" logo, too.
As none of these sold too well in Japan, I guess Technics was left with a stock of unused chassis and made one export version to make room for the immediate future instead of stacking the past.