Hi Neil, sorry but they do tell you that. See pic! Extract from the pdf file data on the site, which one can read before ordering!
Check this out-
http://www.componentshop.co.uk/pdf/P94.pdf
You need the slots for the heat sinks cos you seem to have the 20A version.
Which is more than you need for the Richardson I think.
I don't need the slots cos I bought the 10A P94 Lite version.
What do we learn? Slow down and read the manual and small print 😉
Cheers Doug 😎 Have a good break. Kit-Kat? 😉