i'm pretty sure it's a 6 speed cassette, though it could have spider cogs on it. it's a shimano 600 hub with indexed cogs and not dura ace. I've found 6 speed cassettes on ebay supposedly by shimano, but don't feel like spending the money for them. i tried to use the lockring tool because there was a splined shape hole inside the hub, but the tool would not fit inside.
The shimano 105 hub i have has threads on the end of the free hub, so i assumed that the lock ring was the last cog.