The only 617 with an alloy cylinder was the S&W 617-2, which was a 10-shot cylinder.
Edit: copied from another post:
617 no-dash: forged hammer/trigger, 6-shot, non-drilled&tapped frame
617-1: forged hammer/trigger, 6-shot, new-style rear sight leaf w/ drilled & tapped frame
617-2: forged hammer/trigger, 10-shot, new-style rear sight leaf w/ drilled & tapped frame (all later 617s have the new-style rear sight and D&Ted frame). The cylinders on these started out aluminum alloy; by the end of the -2, S&W had figured out how to make a 10-shot stainless steel cylinder and were shipping 617-2s with SS cylinders.
617-3: MIM hammer/trigger, 6-shot
617-4: MIM hammer/trigger, 10-shot
617-5: same as 617-3, w/ Internal Lock System
617-6 (current production): same as 617-4, w/ Internal Lock System