panel comes with a set of 19" mounting brackets and a set of 23" mounting brackets, so the replacement part would depend on which mounting option you are using.
For 19" mounting brackets, you would need to order:
Qty 1 - 115278-1RC
Qty 1 - 115279-1RC
For 23" mounting brackets, you would need to order:
Qty 2 - 115280-1RC
The replacement brackets do not come with the set screws, which are 8-32 screws that are 0.375" long.