Yes, as per above responses, suffix 'S' (w/o any other letter) stands for stainless. Available sizes given in Table 10-18 Properties of Steel Pipe, Perry's, 7th Edn.. Footnote:
5S, 10S and 40S from Stainless Steel Pipe, ANSI B36.19.
I wish to point out that S in combination with another letter is not stainless: ST = standard wall, XS = extra strong wall, XX = double extra strong wall, and Schedules 10 through 160 from Wrought Steel and Wrought Iron Pipe, ANSI B36.10.