Apple Macintosh Performa 6400 Series - Manual (Page 141)

Some communication cards designed for other communication slots are not
compatible with your computer and cannot be installed in its communication
slot. You can tell by examining the placement of the notches on the card. Use
the following illustration as a guide if you’re not sure you have the correct
type of card for the slot.
Vertical plate
Compatible
Compatible
Notches
Logic board
(side view)
Communication slot
Communication card
Notches
Not compatible
Notches
141 Installing Expansion Cards