MX2020s, MX10K3, MX10K4  show that the RE has a serial number and appears like it is a modular single part.

Routing Engine 0 REV 07 750-073158 ABC123 RE-S-2X00x8

Routing Engine 1 REV 06 750-073158 ABC123 RE-S-2X00x8

CB 0 REV 11 750-073160 ABC123 MX2K Enhanced SCB

CB 1 REV 12 750-073160 ABC123 MX2K Enhanced SCB

They are one single part. Often called "RE-CB" or "RCB"

https://www.juniper.net/documentation/us/en/hardware/mx2010/topics/concept/mx2010-host-subsystem-description.html

The CB-RE is a combined Routing Engine and Control Board in one unit. 

https://www.juniper.net/documentation/us/en/hardware/mx2020/topics/task/maintain-mx2020-routing-engine.html

Each host subsystem comprises a Control Board and Routing Engine (CB-RE) functioning together.

For RMA purposes, the CB SN is used. You can ignore the RE SN.