Clip Zip Firmware Version

Hi:

I've put rockbox on clip zip's before and have got the version 18 
firmware from Sansa and successfully installed Rockbox.

I've just got a new Clip Zip that has version 21 firmware (system info 
v01.01.21A) and I've been reading here about problems with using the 
correct firmware, essentially that 21 isn't supported by the 
installation process but 20 and 18 are.  So what version should I use? 
The manual still says version 18.

Thanks
Sheldon
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