After SunTour went into a long slow decline, it transmuted into SR SUNTOUR and moved from Japan to Taiwan. And throughout the 1990s the derailleur range gradually decayed into a mish-mash of low-end, sub-Shimano steel and plastic.
But the arrival of the new millenium seems to have re-energised SR SUNTOUR, which suddenly produced a whole set of aspirational, innovative and freaky looking derailleurs. These featured straight cable runs, and many had a unique ability to be compatible with both Shimano and SRAM levers. They achieved this seemingly impossible feat by cunningly having two possible ways you could run the cable on the one derailleur.
I think that this handsome item is a SR SUNTOUR Duro (720 GTB), however it has a different design of pulley cage from my SR SUNTOUR Durolux GTB, so these letters may not be correct. I have guessed that 'GTB' refers to the length of the pulley cage, rather than its look. The '700 series' number may indicate that both knuckles are aluminium. The SR SUNTOUR Duro (720 GTB) featured: