I feel IOS it gets negotiated to RPR mode
Error Message %ISSU-3-PEER_IMAGE_INCOMPATIBLE: Peer image (%s), version (%s) on peer uid (%d) is incompatible
Explanation The peer client is incompatible.
Recommended Action Synchronize the configurations and reload the standby supervisor.
Error Message C4K_REDUNDANCY-6-MODE:[char] supervisor initializing for [char] mode
Explanation The supervisor that you are not directly connected to is currently initializing as either the active or standby supervisor, in RPR mode.
Recommended Action This is an informational message. No action is required.