Re:
I enclose chart from campagnolo
hope this explains things
Z.11 is for cassettes where the small sprocket has 11 teeth (the outside diameter being smaller)
I think my CS301 is the one Clemenzo in the link above. It is alloy and for 11t.
The CS201 is not in stock from anywhere.
Someone in USA has a titanium one on ebay!!! ouch.
I have only one set of wheels that requires 26mm size and at present has a 11t cassette.
I have loads of the 27mm ones mainly for 12t and over
26mm I have one CS301 fitted and one spare CS201 and CS301(alloy)
The chart is available from campagnolo spare parts and tools catalogues that can be found easliy in the early 2000s time period.
Tim