IBM 3270 Maintenance Manual page 635

All Points Addressable Adapter
Replacement
1.
The all points addressable (AP A) adapter
must be placed directly adjacent to the
display adapter; the display adapter retaining
bracket has been stamped for easy
identification (see below).
2.
Position the display adapter so its front edge
lines up with plastic card guide in the front of
the system unit and the contacts on the
bottom edge line up with the socket on the
system board.
5-3200-10 Maintenance Information