That's not an SIS rear derailleur. SIS only came out in 1985 and that year it was only on Dura-Ace. For 1986 it trickled down to New 600EX and Light Action, but it was an option, as many bicycle manufacturers still weren't convinced of its potential. Miyata decided to spec the friction option on their 600 and Light Action models in 1986. The pictured derailleur is the non-indexed RD-L523-SS. The indexed version, which is the RD-L525-SS, looks very similar and it's easy to confuse them.