Precision Funnel 58:
If you own a machine model from the following manufacturers and use their standard filter baskets, the Precision Funnel 58 fits perfectly: 

Bezzera, BFC, Brasilia, Carimali, Conti, ECM, Elektra, Faema, Fiorenzato, Gaggia, Grimac, IIsomac, IZZO, La Cimbali, La Marzocco, Nuova Simonelli, Quickmill, Rancilio, Rocket Espresso Milano, Vibiemme, Wega

Precision Funnel 41:
The Precision Funnel 41 was specially developed for the Precision Tamper 41 and can only be used with it. Together they perfect your espresso preparation when using the LM 7g Strada filter basket. The La Marzocco LM 7g Strada filter basket is required for use.

How do I know if I have a 58mm filter basket?
The 58mm is the inside diameter of the filter basket. Just take your portafilter out of the brewing group and measure it as shown in the picture. Important: If the inside diameter is just under 59mm, you speak of a 58mm filter basket. The sizes “58,0mm and 58,5mm” fit for these filter baskets.