I've only ever seen them on jeeps and open cab trucks from D-Day to Berlin, but they were quite common on those trucks.
Usually made from one lump of 1.5" angle iron on a Dodge, but if you fit one make sure it stays within the vehicle size envelope when complete so it isn't a hazard to pedestrians.
If you fit it and get Police hassle, remove it, as you can't hide behind the usual "it's been there since it left the factory" excuse as it isn't in the TM's or Parts Lists, as you point out.
Gordon, in Scotland
( Now officially given up on any form of politics )