> I think these might be the ones:
>
> http://www.mouser.com/search/refine.aspx Ntt=550-57001

Rapid in the UK also carry them:

<http://www.rapidonline.com/productinfo.aspx kw=knobs&tier1=Tools%
2c+Fasteners+%26+Production+Equipment&tier2=Fasteners+%
26+Fixings&tier3=Knobs&tier4=15mm+mixer+style+control+knobs&moduleno=6
4708>

That's quite a link, so their home page is:

http://www.rapidonline.com/

and parts nos are:
32-0390
32-0395
etc. up to 32-0415

- black knobs with six different colours of stripe, Rean P570-16
series.

Tim