57mm is the size of Lelit's LELIT57 group and tool line, used on its entry-level machines (Anna, Diana). It is one millimetre narrower than the 58mm commercial standard — 58mm baskets and tampers do not fit, and IMS makes a dedicated B64 basket series ('for tamper 57') for these machines.
New to portafilter sizing? See espresso portafilter sizes explained for how 57mm compares to the other standards and why parts don’t interchange across sizes.