I actually don’t think there is only one ‘Spurs System’.
When Duncan was right out of college they operated mostly from the post and then when David Robinson retired, San Antonio did play slow iso style based on defence and the growing offensive skill of Parker & Ginobilli.
Their last title run was predicated on ball movement because they lacked a true superstar and had lots of high I.Q veterans who like Duncan, Ginobili, Parker, Diaw, Belinelli and to a lesser extent Baynes, Leonard and Mills.
I think the real ‘Spurs System’ is actually sacrifice. It’s by allowing Popovich to build a system based on the players strength and like Belichick’s Patriots, playing within your role which is easier said then done with elite players egos. Pop even said during Duncan’s retirement ceremony that it was Tim that allowed him to succeed by not only being a hall of famer but by sacrificing so much, that the rest of his teammates had to follow suit.