Sorry, I cannot answer your specific question NB
I brought mine from Machine Mart a few years back and I don't think it was that expensive (can't remember now but I do know I won't have paid too much – I'm too tight!). Mine works well on the wheels I use.
To be frank, I didn't know they came in different "Carats" (wheel dressers – not diamonds that is) let alone know which size I should be choosing and I don't think MM offered any choice in that area.
Regards,
IanT