Chris M writes:

Also, there are no set screws and it
looks like the one end being open means I need to push them out
against the direction that the rim surface pushes them.

The easiest way to remove the pad is to insert the head of a flat
screwdriver (a depth of 5 to 10 mm should be plenty) between the
bottom of the pad and the holder from the open end and twist. Use
pliers to push the replacement pad into place.