I haven't seen any others with the all black knobs but I don't see them very often so it could still be original. How does the serial numbers compare? Is it the newest or oldest that has all black knobs?
As far as I can tell, the black knobs are not original.
The original knobs are the ones which match those use on older Roland synthesizers, such as the SH-09. The knobs on these over-looked synths can be lost, thus hijacked from items such as the KM-2 (which could have been useful for keyboardists back during the time of its production). I could see how someone would replace their SH knobs from ones taken from a KM-2, especially as the synths are so much more valuable.