I was reading this article here about cool fast cars that are sort of forgotten about including 420 turbo.

https://www.hagerty.co.uk/articles/vani ... -are-left/

It says they were built to order. I was wondering if that’s true and what does that actually mean? They only bother fitting the turbo engine if some one walks into dealers and asks for it? Or do dealers think I can sell 10 of them and orders them?

That may have been true as it would explain why Rover didn't get close to the planned 500 volume for each of the 200 3 door and 400 4 doors.

I think there were less than 300 of each built. Not cheap either...

Many of the lower volume models were built to order. It made sense that when there was spare capacity to build stock cars, they built cars that would sell. Also you find that if buyers are paying a lot of money for a car, they tend to want the exact colour and spec of their choice.

I don't know about Rover customer ordered cars but Ford used to put an S.V.O. plate on them, usually on the inner wing.

This was a Chassis style plate that stated the requested specifications.

I had a Mk2 RS2000 Custom that was a Special Vehicle Order, it stated Body Colour Black & Trim Colour Tan on the plate.

Rover might have done something similar for build identification & /or individuality. You might even have a number somewhere if these are low volume special models.