Hi again Daz,
"Litter" is the official US Army term for any flat supporting framework, such as a piece of canvas stretched between parallel shafts. As you will notice, all of the designations for stretchers in our database are indicated as "Litter". The origins of the word Litter are actually from a vehicle-type device that was first developed in Asia. It consists of long, often wooden or bamboo poles which are held at each end by a person. Four people are used to transport the litter, using their shoulders to support the weight. Between the two poles is small canvas or wooden compartment in which the passenger is located. This was a very primitive form of carriage, or taxi.
A "stretcher" is, in theory, a simple version of a litter, although I suspect that the idea of a "stretcher" being called a "litter" dated back well into the 1800s, and army tradition etc., meant that the nomenclature of such an item remained the same.
I hope that the response makes sense, if not, kindly let me know and I will try and clarify.