Cisco Part Number: C8231-G2-RM-19= is a Rack mount kit - 19" Fixed for Cisco C8231-G2, spare
The C8231-G2-RM-19 is the rack mount kit option for the Cisco C8231-G2 router. It is used to mount the C8231-G2 device in a standard 19-inch rack. This mounting kit is listed as an option accessory specifically for the C8231-G2 model in the Cisco 8000 Series Secure Routers Ordering Guide