When you look at the Wall and Ceiling Mounting section of the guide, it shows that the spaces between the two pilot holes need to be bigger for the WAP321, most likely meaning that the bracket for the WAP321 is bigger. Even then, looking at images for the brackets, it shows that they are different sizes. I do not think it is possible to switch between mounting brackets because of that. Does this help you?
Currently this is not a separate orderable item within Cisco. I also looked and did not see any compatible 3rd party devices. You might try opening a case and doing a feature request to make those orderable if you like.
Special thanks to Alex McDonald for sharing his favorite tip:My favorite
tip is the ‘write default’ command on autonomous wireless access points,
it simplifies manual conversion of a WAP with a static ip address to a
LAP with a DHCP address.This saved a l...
With the latest release of firmware for the WAP 561, 551, 371, 351 a new
feature was added that would allow customers to upgrade the firmware for
their clustered access points from the main access point.The basic steps
to accomplish this is:1) Download th...