What's the difference between a Cam thrust plate & a cam wear plate?

Okay ladies and gentlemen, from way up in the cheap seats, in the peanut gallery, comes, "Stupid question time!"

What's the difference between a Cam thrust plate & a cam wear plate?

Do you use one or the other?

I am looking at my Year One catalog (R1110) page 252



-cause ignorance is bliss, until there are consequences...