It’s a ’one-legged’ milking stool, a spin-off from the invention of Alfred Nobel who insisted that they were used by his workers when looking after his nitroglycerin vats (Absolut Vodka is not the only good thing to come from Sweden!).
The stool prevented workers from falling asleep on the job and risking falling into the vat. Early health and safety, you might say. Nobel’s design was, of course, not so sophisticated as the one in the picture. A later, simple wooden design was used, not only by milk-maids but by people who like them moved from one workplace to another and were required to sit. Strapped to the rear end, you could take your seat with you while keeping your hands free to carry other essential work requirements – milking pails, for example.
The modern variant is equipped with a spring coil which acts as a shock absorber for the spine.