The Germans introduced a small flamethrower in 1917 to replace the earlier Kleif. The Wechselapparat ("Wex") had a doughnut-shaped backpack fuel container with a spherical propellant container in the middle. This design was updated during the Second World War to become flamethrower model 40. However, model 40 was considered too fragile so it was soon replaced by the model 41, a simpler construction with smaller, horizontal, cylindrical backpack containers.
The doughnut-shaped container design was copied by the British during World War II as the Flamethrower, Portable, No 2.
"Wechselapparat" is German for "exchange apparatus".
References
Wechselapparat Wikipedia(Text) CC BY-SA