There are 2 types of collars for the BES900/XL and the BES920/XL, they are both identical except the internal rim inside the collars vary in thickness, the one in the newer BES920/XL machines have the thicker rim which assists in the portafilter contacting the seal much sooner than the BES900/XL creating a tighter seal.

Some 1st generation BES900/XL models had their whole group head, element and collar upgraded under warranty also.

Ensure you purchase the correct collar for your machine.