A bit of research and I found a picture of one on the rear of a motor on the Searle 'Estate Wagon and Station Wagon' brochure at Hermanns Land Rover Brochure page.

Then earlier on this week I saw one on Ebay:
TBH it looks like a normal jerrycan holder for the lights in wing models with the addition of 3 brackets. The 2 x bottom brackets seem to fix where the grab handle normally goes, and the top bracket is attached to the rear quarter panel:
Front view

Side view

I can't see a way of it being usable if you have a bottom awning rail but if you haven't they might be another solution to carrying more fuel!