The Puly Blindy 58mm, ideal for E61 Portafilters and heads + others which are:
Most Astoria (check before buying), Casadio, Conti, most Elektra (check before buying), Fracino, Gaggia, La Cimbali, La Marzocco, Magister, Nuova Simonelli , most Pavoni commercial machines (check before buying), Promac, Rancilio, Slayer, Synesso, Victoria Arduino, ecc.
Machines with e61 equipped heads from the manufacturers: Ascaso, Bezzerra, Bfc, Bianchi, Brasilia, Ecm, Expobar, Faema, Fiorenzato, Grimas, all E61 Iberitals (Lanna, Junior, Ladri Classic, New Ladri, some but not all Expressions (check before buying), La Nuova Era, La Scala, Quality Espresso, Rocket, Royal, Sabitali, San Remo, VFA, Vibiemme, most Wegas (check before buying).
The blindy is a deep dish backflush basket, having a deeper dish gives the powder more time to dissolve before the pressure takes it up into the internals of the espresso coffee machine to clean it.
It has been cleverly designed so that the line sat in the bottom of the basket indicates when you have the correct level of puly caff cleaner, if free pouring from the 900g tub.
These are the best you can get and are highly recommended by Happy Donkey, they work so well.