For a majority of E61 home espresso machines, you'll need the 8.5mm group gaskets.
Some machines that we carry which use the 8.5mm group gasket are:
Izzo Vivi, Izzo Alex Duetto, Rocket R58, Rocket Mozzafiato, Rocket Appartamento, Rocket Giotto, Quick Alexia, Quick Mill Anita, Quick Mill Andreja, Quick Mill QM67, Quick Mill Vetrano 2B, Quick Mill Carola.
All 3 of these sizes are compatible, it depends on which thickness you prefer. Upon initial installation you will notice your portafilter does not line up at the "6 o'clock" which is completely normal.
When replacing gasket remember the flat side is facing down.