Merc designation #'s question

Joined
Aug 27, 2011
Location
MN
#1
I constantly see reference to the numbers on Merc props and have no clue what they tell us. A45, 46, A47??

My current Trophy Plus 24 pitch is a A40?? Some people ask for these numbers specifically when searching for a used prop and I'd like to know why. I've read comments alluding to blade thickness, is this all it is? They don't look that different to me.