You need to collect the serial number of an SRX Series device to replace it.

The correct command to collect the serial number of an SRX Series device is the show chassis hardware command [1]. This command will return the serial number of the device, along with other information about the device such as the model number, part number, and version.

This command is available in Junos OS. More information about the show chassis hardware command can be found in the Juniper Networks technical documentation here [1]: https://www.juniper.net/documentation/en_US/ junos/topics/reference/command-summary/show-chassis-hardware.html.