There's thread upon thread about the differences on the s1 forum.
No s1 should have short gutter rails,all rs1600is have short gutter rails,no argument.
Think the change from short to long was between end of production of the rs1600i in 1984 and the start of s1 production in late 84/85.
Nobody can exactly say why they were changed as rs1600is sometimes had opening 1/4s as an option but still had short rails.
Easiest way to spot a reshelled/fingered rs1600i/s1 is the rails.