Is this chape really so rare and does anyone know about it?

The "high" and "low" refers to plain (V-100) or deluxe (V-200) trim, not to any physical aspect of the car.

This is not a standard wagon—it reminds me a little of the funerary car (hearse) conversions to Spanish Dodge Dart wagons, but this looks more like some kind of a special delivery or service vehicle.

Given the great difficulty of making such modifications after the car was built, it was most likely built by special order at AMAG, the Swiss builder of Valiants at that time.

Interesting car! Will you show us more pictures of the unusual back end of it?