Telect offers the rear support brackets for securing an end channel to the backside of an unequal flange rack in different lengths to help accommodate the use of a capacity extender before the end channel.  The rear support brackets come in different lengths depending on the width of the capacity extender being added to the unequal flange rack.  The different part numbers are:

For a 2.5" wide capacity extender, you would use the 02128E01+15 rear bracket spacer bar.

For a 5.0" wide capacity extender, you would use the 02128E02+15 rear bracket spacer bar.

For a 7.5" wide capacity extender, you would use the 02128E03+15 rear bracket spacer bar.

For a 10.0" wide capacity extender, you would use the 02128E04+15 rear bracket spacer bar.

The part number is for quantity of 1 ea, and does not include the support hardware.  For a 7ft end channel you would need a total of 6 rear bracket spacer bars, and can reuse the mounting hardware included with the end channel.