Originally Posted by
HillRider
This is correct. The 105 9-speed cassettes have all full-plate cogs held with removable bolts and can be used individually. Ultegra 9-speed cassette have the #5 and #6 cogs on a spider and the three largest cogs on a single spider and they can't be used individually.
Typical Ultegra 9-speed cassettes have the 5th and 6th cog pinned together but the splined centers are part of the cogs and can be used as individuals if you can separate the cogs from the pins. The larger cogs that are pinned to a common carrier cannot be used as individuals. The 9-speed Dura-Ace cassettes that I've seen have the 5th and 6th cogs pinned to a common carrier, like the larger cogs, and cannot be separated and used as individuals.