Thanks everyone for the information.
I'm working on the basis that my car was probably pre-registered by Rover, was part of their ACOP programme for employees, or was a management car.
There's a post on mg-rover.org that mentions ACOP cars were registered via the Kettering DVLA office and would therefore have had Northampton suffixes or pre-fixes (depending on when it was registered). I've been told by a former Rover employee that the Rugby preparation centre was used from the mid-1990s until the Rover/Land Rover split.
"ACOP/Employee Cars were not registered in Birmingham, most were done in Kettering (Northampton) as it was the closest office to where the cars were PDI'd hence the "KV" or "K?" plates.
The vast majority of Engineering cars were registered in Birmingham, "BX" plates as they didn't require PDI as we used to do them all in the Flight Shed or in Methods build."
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https://www.mg-rover.org/threads/who...direct.281602/