Apple Macintosh 6500 - Manual (Page 205)

6 Supporting the adapter fence with one hand, push the card with the other hand until it
snaps into place.
Don’t force the card. If you meet resistance, pull the card out and try again. To
see if the card is properly connected, make sure its metal fence is flush with
the side of the adapter fence.
Adapter fence
Card fence
205 Installing Expansion Cards or Additional Memory