What is a 'Rhodesian boiler'?

My first thought was, why are these domestic hot water boilers called 'Rhodesian boilers' throughout most of British colonial Africa? And what exactly makes a boiler 'Rhodesian'? Elsewhere these types of boiler are usually referred to as 'donkey boilers' - in lieu of any other leads, I thought I might start my research there.

The Rhodesian Boiler

The Rhodesian boiler I built for my parents during my teens is finally falling down. Although almost 40 years old, its condition hasn't deteriorated to quite the same extent as this one:

 I have constructed a few more of these since, but thought that this time around I should do some research on the web beforehand (I didn't have that luxury in those days.) I found a number of photos of the classic design, as found around many rural houses and camps throughout British colonial Africa:

To my surprise, however, there was very little other technical information available and, as it appears that I know more about these boilers than most other folk, I decided this time around to write up my experiences on this blog. So, here goes ...