Rust R has been frustrating me for quite a while, as I was fairly sure I had identified it as a BSH, a French kit car intended to be based round a Renault 8 Gordini as donor car. The initials are from the surnames of François Benais and Max Saint-Hilaire, who produced the kits. Up until now, however, I've found it impossible to make any connection to any of the Water entries.
One thing I learned early on while Autopuzzling is that you often have to use search terms in different languages, given the international nature of the membership and the subject material. I don't know why I didn't do it before, but assuming that the car would be in France, and quite likely be for sale, I searched using "BSH voiture à vendre", and found the actual car, and very instructive it was too.
It seems that the first owner, a M Guiry, included his own name when registering the vehicle, so the documentation shows it as a BSH Guiry.
From there it's a short leap to Water 21, a water pump in Guiry-en-Vexin, in the Val d'Oise in northern France, and from there to Extension b, electric all-terrain vehicles which can be hired at the Château de Guiry for use by visitors with reduced mobility.
So Rust R: BSH Guiry - Water 21: Water pump at Guiry-en-Vexin - Extension b: All-terrain mobility vehicles at the Château de Guiry-en-Vexin.