As a Pressed Steel Class 121 is effectively a Class 117 DMBS with an extra cab, it was a natural choice to replace the DMBS vehicle of a 117 set if required for maintenance purposes. Such is the case here, as 55022 (L122) is deputising for 51358, which has been removed from L420, leaving just TC 59510 & DMS 51400. The ensemble is pictured passing South Stoke on 27 May 1989 with the 5C68 17:30 Didcot Parkway to Reading Network SouthEast ECS. As the headcode implies, this was the empty stock for the 2C68 18:10 Reading to Oxford service.