The Macintosh 500 family has a processor direct slot (PDS) for expansion. While Apple does make a Nubus adapter card to allow a Nubus card to be used with the PDS in other computers, the design of the Macintosh 500 series does not allow enough room to insert the adapter card.
Macintosh computers the 500 family include:
Macintosh LC 520
Macintosh LC 550
Macintosh LC 575
Macintosh LC 580
Macintosh Performa 550
Macintosh Performa 560
Macintosh Performa 575
Macintosh Performa 577
Macintosh Performa 578
Macintosh Performa 580